Outdoor wood finishing: A review on making wood resistant to moisture, ultraviolet light, and degradation

Abstract

This article reviews published literature related to the coating of wood surfaces for external applications. Research has shown that a wide range of procedural steps and components in coating formulations can contribute to increasing the effective service life of the coating as well as to maintaining the quality of the coated wood surfaces. Published findings support the idea that the commonly observed service life of painted wood surfaces exposed to outdoor weather can be significantly increased by dedicated application of such measures as optimized sanding, the use of an effective primary coat, the type of resin in the finish coat, increasing the number of layers of the finish coat, and a wide range of issues related to formulation of the finish coat. Even if a majority of contractors and homebuyers continue to prefer such options as vinyl or aluminum siding, the market opportunities remain very large for clients who prefer to rely on coatings and wood products for exterior surfaces of buildings and other exterior wood items.

Full Article

Outdoor Wood Finishing: A Review on Making Wood Resistant to Moisture, Ultraviolet Light, and Degradation

P. Frederik Laleicke  and Martin A. Hubbe  ,*

This article reviews published literature related to the coating of wood surfaces for external applications. Research has shown that a wide range of procedural steps and components in coating formulations can contribute to increasing the effective service life of the coating as well as to maintaining the quality of the coated wood surfaces. Published findings support the idea that the commonly observed service life of painted wood surfaces exposed to outdoor weather can be significantly increased by dedicated application of such measures as optimized sanding, the use of an effective primary coat, the type of resin in the finish coat, increasing the number of layers of the finish coat, and a wide range of issues related to formulation of the finish coat. Even if a majority of contractors and homebuyers continue to prefer such options as vinyl or aluminum siding, the market opportunities remain very large for clients who prefer to rely on coatings and wood products for exterior surfaces of buildings and other exterior wood items.

DOI: 10.15376/biores.20.3.Laleicke

Keywords: Exterior wood; Wood coating; Protection; Weathering resistance; Antimicrobial treatments; Moisture protection; Paint; Varnish

Contact information: Department of Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC, 27695-8005 USA; Corresponding author: hubbe@ncsu.edu

INTRODUCTION

According to the National Association of Home Builders, most single-family houses in the US are being sided (or “clad”) with such products as vinyl siding (25.6%), fiber cement (21.7%), or brick-like materials (18.5%), leaving only about 5% of wood-clad houses (NAHB 2024). Back in 2000, wood comprised about 13% of the share. For the wood industry and the paint industry, this circumstance highlights both a challenge and an opportunity. On the one hand, many builders and homeowners have come to the conclusion that non-wood siding represents a better investment than painted wood exteriors. But on the other hand, painted wood still represents a major category that can offer attractive prices, workability, and a sense of familiarity. This review article considers factors affecting the durability of outdoor-applied finishes for wood surfaces.

Goal: Protection of Exterior Wood

The damage that can happen to wood that is exposed to outdoor weather, including mold and insect attack, has been covered in earlier review articles (Feist 1996; Kropat et al. 2020). For instance, it is well known that exposure of wood to ultraviolet light, in combination with periodic rain, will result in weathered wood, which involves depletion of lignin in the outermost layer (Williams et al. 1996; Kropat et al. 2020; Gurleyen 2021). Cycles of repeated wetting and drying of wood that is exposed outdoors can be expected to develop periodic cracks, which are known as checking (Nejad and Cooper 2011; Lestari et al. 2020). Boards exposed to periodic rain, followed by drying, also can develop cupping, in which the last-dried side may shrink in comparison to the rest of the material, leading to a concave shape exposed outwards (Fufa et al. 2012).

In cases where wood remains moist for a long time, or if it is near to soil, mildew and rot may proliferate (Bjurman 1992; Brischke et al. 2006; van Meel et al. 2011). Fungal species typically play a leading role in the decay of wood (Goodell 2003; Cogulet et al. 2018a), though bacteria also are expected to be involved (Clausen 1996). Wood that is near to soil also can be subject to attack by termites, which chew tunnels into the wood and build nests (Remadevi et al. 2015). In cases where the wood surface has been covered by a coating layer, exposure to outdoor conditions may eventually lead to flaking (Williams and Feist 1993), after which at least a portion of the wood becomes directly exposed to the weather.

Hypotheses

For the purpose of focusing attention on some important issues, a series of hypotheses, as follows, will be considered in this article (Fig. 1):

1. The durability of the coating layer itself, including its resistance to cracking, flaking, and peeling, is of paramount importance, since without such strength and adhesion to the wood surface, it will not remain present to provide its other functions.
2. Finish formulations for outdoor application need to contain a component capable of absorbing ultraviolet light, so as to protect the lignin in the outermost wood layer (about 1 mm) from photochemical degradation (Williams et al. 1996).
3. Finish formulations for outdoor application need to contain sufficient hydrophobic resin component to prevent liquid water (from rain) from contacting the wood directly.
4. Biocidal, including antifungal, antibacterial, and insect-resistant treatments can play significant roles in extending the useful lifetime of wood finish in outdoor applications.
5. The formulation of a wood coating for outdoor applications can significantly affect its outward appearance both initially and during its period of use.
6. By investing more in the quality of wood preparation and in the composition of the coating layer formulations, there is potential to greatly increase the length of time during which a wood coating can effectively protect wood that is exposed outdoors.

Fig. 1. Some hypotheses regarding how exterior wood can be better protected by improvements involving coating technology and its optimized application

Published Reviews: Finishing for Wood Protection

Table 1 lists some key themes of review articles and books that have dealt with various aspects of outdoor wood finish applications.

Table 1. Review Articles and Books Dealing with Aspects of Outdoor Wood Coatings

The first six items listed in Table 1, covering a period from 1978 through 1996, all were written by researchers at the Forest Products Research Laboratory of the Forest Service, a division of US Department of Agriculture. Review articles from other sources dealing with outdoor application of wood coatings did not begin to appear until 2004, though there has been an acceleration in the most recent decade.

BACKGROUND OF DAMAGE TO OUTDOOR WOOD

Overview of Wood Weathering and Decay

Within the category of wood weathering, this review article will be ultimately concerned with what has been called “natural weathering” of coated wood, i.e., that which occurs under typical outdoor conditions (Feist 1982a, 1983, 1988, 1994). This is an important distinction, since many similar effects can be achieved by artificial or “accelerated” weathering and by various approaches intended to simulate the effects of weathering relative to the outward appearance of wood surfaces (Kropat et al. 2020). The purpose of accelerated weathering is to enable more rapid and reproducible testing, especially in the case of newly developed coatings for outdoor applications of wood.

Natural weathering

As noted by Cogulet et al. (2018b), wood weathering can be attributed to the combined effects of light irradiation (especially the ultraviolet component), water, oxygen, temperature, and the colonization of wood by fungi. Striking photographs of weathered wood have been presented by Williams et al. (1996). One of the most noticeable effects of weathering is a change of color. Often the color undergoes a two-stage transition, initially acquiring a redder tone, and thereafter tending to become grayer and bluer (Kropat et al. 2020). The first part of this transition is understood to involve changes in the molecular structure of the lignin component of wood, such that the absorption of light in the visible range is increased in transient ways. The second part has been generally attributed to the gradual depletion of lignin in the outermost 1 mm or so of wood, due to its photochemical breakdown, followed by the leaching action of water. The color of weathered wood usually is also affected by the colonization of the surface by blue-stain fungus, which takes its name from the color of the fungal hyphae.

Artificial weathering

Although resistance to genuine outdoor conditions is the true test of a wood finish, weather conditions are subject to huge variability. Artificial weathering equipment and procedures can overcome such problems, but the outcomes can be expected to show some characteristic differences from natural weathering. In particular, wood that has been exposed to cycles of ultraviolet light and water spray in a chamber generally will not experience any fungal infestation (Kropat et al. 2020). On the other hand, artificial weathering tests have made it possible to evaluate a wide range of wood coatings under controlled conditions (Cogulet et al. 2019). For instance, Cogulet et al. (2019) were able to demonstrate that ultraviolet light plays a critical role in the weathering of wood. Likewise, de Mesquite et al. (2020) were able to document changes in lignin structure, by means of infrared spectrometry, following artificial weathering. Hinderliter and Sapper (2015) used accelerated weathering tests to show differences in the permeability of wood coatings, thus affecting the rates of migration of moisture. Podgorski et al. (1994, 1996) used artificial weathering as a means of studying the changes in glass transition temperature of alkyd resins of wood coatings in the course of exposure.

Simulated weathering

By means of staining, mechanical abrasion, and other such treatments, it is possible to render wooden surfaces more suitable for certain kinds of displays, which may or may not resemble genuinely weathered wood. For example, wood pieces subjected to simulated weathering may help to establish a mood or a theme in an indoor setting. This type of treatment has been studied based on social media posts (Kropat et al. 2020).

General effects of finishes on weathering

As a general trend, the application of a conventional wood finish can be expected to greatly diminish weathering effects, at least during the recommended lifetime of the coating (Feist 1982a, 1983, 1988, 1990, 1994). In addition to the effects of weather on exposed wood, the term weathering also can be applied relative to changes in the coatings itself in the course of exposure to weather. A prime example is the depletion of binder, i.e. “chalking,” in certain wood finishes over the course of exposure to ultraviolet light, rain, and general exposure (Allen 1984; Miklecic and Jirous-Rajkovic 2011; Nguyen et al. 2018). This phenomenon and its most likely causes are illustrated in Fig. 2. As noted, ultraviolet (UV) light is expected to gradually break down the binder. The “chalk” that tends to come off onto one’s fingers when inspecting the surface likely comprises mineral components of the coating, e.g. TiO₂ or CaCO₃. In addition, depending on the porosity of the wood, some of the binder may have immediately migrated into pores of the wood (i.e. vessels or tracheids), thus leaving an insufficient amount of binder within the coating layer.

Fig. 2. Illustration of some contributions to chalking of paint layers exposed to weather

Overview of Wood Decay

What would happen if wood didn’t decay? According to Schwarze et al. (2000), there is a dynamic balance between the growing amount of woody biomass and its breakdown in the natural environment. Decomposing communities (Kahl et al. 2017) of biotic agents such as fungi, bacteria, and insects are the main decay agents that decompose holocellulose and lignin. By breaking down these main chemical components of the woody structure, the nutrients are made available to other organisms, eventually allowing new growth of plants. In the built environment, however, the decay of wood affects the longevity of wood structures and requires protection of the wood by physical and chemical means. Table 2 lists review articles dealing with various aspects of wood decay.

Table 2. Review Articles and Books Dealing with Aspects of Wood Rot and Decay

Upon the inspection of wooden structures, such as a fence pole, an attic, or a standing tree, one can come across three different types of rot: brown rot, white rot, and soft rot (Worrall et al. 1997). Different decay fungi cause them, each with their individual types of chemical and enzymatic degradation (Veloz Villavicencio et al. 2020), conditions of the remaining wood, and impacts on its mechanical strength. The main reason for this multiplicity of enzymes is that cellulose, hemicellulose, and lignin require different degradation pathways. Different fungal species prefer specific hosts due to their differences in composition, such as the types of lignin in softwoods vs. hardwoods (Tuor et al. 1995). For fungi to thrive on a digestible substrate, they require a suitable combination of moisture (Thybring 2017), temperature, oxygen, and pH. According to Zabel and Morrell (2012), fungi need free water, some atmospheric oxygen, and temperatures between 15 and 40 °C. This is supported by a review of the degradation mechanisms of brown rot by Ringman et al. (2019), who describe how enzymatic degradation requires water for transportation of important metabolites.

Soft rot

Soft rot is caused by deuteromycetes, a.k.a. Fungi Imperfecti and Ascomycetes (Goodell et al. 2008, Schwarze et al. 2000, Lee 2000, Worrall et al. 1997). This type of decay occurs in wet or aquatic environments (Hale 1986). The fungi cause decay at the cellular level and attack the S2 layer of a cellulosic fiber, thus creating a chain of cavities along the hyphae in a helical microfibrillar angle. Upon microscopic observation at the longitudinal face, cavities appear diamond-shaped (Anagnost 1998). Soft rot fungi degrade cellulose and hemicellulose and rarely the lignin (Schwarze et al. 2000).

According to Anagnost (1998) there are three types of soft rot: regular type 1, diffuse type 1, and type 2. Not all types are caused by the same soft rot fungi. Type 1 was observed only on pine and birch, whereas type 2 was limited to pine. Major differences are related to location and types of cavities and erosion caused by the fungus in the wood. While type 1 describes the regular progression of hyphae and cavities within the S2 sublayer of the cell wall, diffuse type 1 accounts for a more diffuse and irregular structure. Type 2 describes soft rot in hardwoods, in which the hyphae penetrate the lumen with significant damage of the cell wall (Anagnost et al. 1994).

Brown rot

Brown rot is the most common and most destructive of the three types of rot (Green and Highley 1997). It is more commonly found in softwood stands and therefore predestined for the decay of structural softwood wood products (Arantes and Goodell 2014). The result of brown rot is a brown, wood-structured residue that has lost all of its holocellulose. The remainder are mostly modified lignin residues and extractives (Arantes and Goodell 2014). The main mechanism consists of the degradation of polysaccharides as well as partial oxidation of the lignin (Worrall et al. 1997; Jensen et al. 2001).

Brown rot causes weight and strength loss. The decay process commences with the degradation of the cellulose, resulting in significant strength loss (Arantes and Goodell 2014; Curling et al. 2002). Subsequent weight loss can be observed at later stages of the decay process. Curling et al. (2002) showed that even at low weight losses, specimens of southern pine exposed to brown rot quickly lost their resistance to plastic deformation in bending at a rate of 3.6% per day. The onset of decreasing modulus of rupture (1.6% per day) and modulus of elasticity (0.6 % per day) were delayed.

White rot

White rot fungi primarily degrade the lignin, but they can also degrade the cellulose (Worrall et al. 1997). For visual identification, the leftover residue has a bleached look (Goodell 2008). Some white-rot fungi are better at removing just the lignin, as mentioned by Blanchette (1991) in his review article about the delignification by wood-decay fungi.

There are two types of white rot: Simultaneous white rot and selective white rot. The first enables the simultaneous degradation of cellulose, hemicellulose, and lignin. The latter is the case when only lignin and hemicellulose are degraded and the cellulose remains untouched (Goodell 2008). Therefore, the residues have a white appearance.

Bacteria

Based on thorough reviews of the roles of bacteria in wood decay by Johnston et al. (2016) and Clausen (1996), fungal decay has been much more explored than bacterial decomposition. Clausen (1996) dates the initial recognition of a relationship between wood and bacteria back to the 1950s and 1960s. Bacteria play an important role in the decay of wood and often work in conjunction with fungi. Johnston et al. (2016) describe the dynamics and complexity of the types and numbers of bacteria throughout the decay process. The number of bacteria, for example, increases as the wood decays.

Greaves (1971) grouped wood-inhabiting bacteria into four main groups. The groups and their characteristics are shown in Table 3.

Table 3. Grouping of Wood-decaying Bacteria and their Characteristics

Groups 1 and 2 impact the permeability of the wood for liquids to more easily move through the wood, with Group two degrading the cell walls. The strength, however, is only impacted by bacteria in Group 2. Bacteria in the third group contribute to the overall decay of the wood as part of the total amount of microorganisms. Bacteria in the 4^th group are passive and support other organisms in the wood with no direct contribution to the decay.

Overview of Insect Damage

Live and dead wood can provide a habitat for many insects. In the natural environment, so-called xylophagous insects are part of the lifecycle of trees and woody biomass.

Table 4. Common Wood Deteriorating Insects and their Damages (Amburgey 2008)

They consume wood at some or all of their life stages. Other insects use wood as a temporary or long-term habitat, rather than feeding on the wood substrate. Sometimes, during imbalances in forest systems, one can observe calamities in which growing populations of insect species can affect, destroy, and eventually help renew large-scale forests. In the built environment, the presence of insects can cause damage to wooden structures and objects.

It is possible to distinguish three main groups of wood-destroying insects: Coleoptera (beetles), Hymenoptera (bees, ants, wasps), and Isoptera (termites) (Unger et al. 2001; Amburgey 2008). Table 4 provides an overview of some common wood-boring beetles. Individual species prefer specific substrates and leave characteristic boring and frass patterns. Unlike in the case of fungi, the wood does not have to have elevated moisture content for some insects. Dry-wood insects, for example, are known to infest in-service furniture, building components, and artifacts used in dry, interior environments.

Seasoning, i.e. the exposure of wood to elevated temperatures, typically achieved during kiln drying, is known to prevent initial and subsequent infestations with some species. As early as 1921, Craighead and Loughborough investigated the fatal temperatures for red-headed ash borers infested ash. The experiments showed that the effectiveness of kiln drying depends on RH, temperature, and time. Temperatures could range from 105 to 135 °F (40 to 57 °C) and eliminate all larvae within an hour at low RH. Heat treatment at even higher temperatures is also discussed in a later chapter, in which the purpose was to improve the dimensional stability of the wood. So-called phytosanitation is commonly practiced to control the spread of plant diseases, including insects, in commercial goods. Invasive alien species (IAS) have damaged and can create significant damage to ecosystems and the built environment (Ormsby 2022). Treatments, per national and international regulations, may involve individual methods or combinations of heat, fumigation, and chemical applications (Leal et al. 2010; Allen et al. 2017). Payette et al. (2015), for example, studied the efficacy of microwave irradiation in the context of packaging materials. Slahor et al. (2005) assessed the efficacy of hot water baths. Sohi et al. (2016) used ultrasonic energy to generate heat at temperatures near 70 °C in pine wood. An abundance of literature sources focuses on traditional and alternative methods to balance the effectiveness of treatments, cost, and environmental concerns.

Termites

Termites are commonly found in subtropical and tropical regions, as they prefer relatively warm environments (Kalleshwaraswamy et al. 2022). They can be found throughout the United States, except for Alaska (Peterson 2006). They are divided into five different groups: subterranean, soil-feeding, dry wood, damp wood, and grass-eating (Olaniyan et al. 2015). All of them feed on lignocellulosic materials (Scharf 2020). Subterranean and drywood termites are efficient agents in wood decay, and they can cause significant damage to wooden structures. In the US, only drywood, dampwood, and subterranean termites can be found (Peterson 2006). Infested wood will sound hollow and dull when hit with a hammer (Gouge et al. 2009).

Drywood termites

Drywood termites live in the southern parts of the United States, between North Carolina and the Gulf Coast, as well as the coastal areas of California (Gouge et al. 2009). As their name suggests, they live in sound, dry wood. Their excrement is dry pellets, a common form of identification as they accumulate outside of their entrances. They feed on early and latewood, following the grain of the wood. Their major feeding source is the cellulose in the wood.

Dampwood termites

Dampwood termites are more adapted to colder climates (Lacey et al. 2010). This relatively small group of termites lives in galleries of moist and decaying wood (Harris 1955) of living and dead trees (Haverty 2003).

Subterranean termites

Subterranean termites live in underground nests. A typical identifying feature is so-called run-ways made of carton material used for transporting food (Harris 1955). Similar to ants, termites form colonies with large numbers of workers, from less than a thousand to 2,500,000 (Ewart and Cookson 2014). According to Scharf (2020), most termites feed on dead wood, and only a few prefer living trees.

Identifying the types of wood that are resistant to termite infestations, construction principles, and treatments is a major focus in the termite-related literature. To prevent subterranean termites, which live underground, from entering structures, preventing access is a major strategy. Ghaly and Edwards (2011) propose the use of rammed sand and soil, concrete and steel to harden the exterior structures of homes against termite infestations and to create impenetrable barriers. This is in line with Peterson (2006), who describes the construction of a home as the best time to prevent future infestations. Chemical prevention, however, is an important factor in mitigating the immense damage of termite attacks.

Wood Treatment Options to Guard Against Rot and Insects

While the present article focuses on the role of wood coatings, it is important not to overlook measures involving impregnating of the wood or otherwise treating it as a means to control against various forms of rot, termites and other wood-attacking insects. Table 5 lists some review articles and chapters that have covered those areas.

Table 5. Review Articles and Books Dealing with Wood Treatments to Resist Rot Organisms, Termites, and Other Insects

FACTORS AFFECTING WOOD FINISHING FOR DURABILITY

Homeowners who are faced with the prospect of repeatedly repainting exterior wood have a strong motivation to invest in some form of siding or cladding, such that they can greatly extend the time before further repair is needed. According to Williams et al. (1996), a typical wood coating, applied outdoors, will last two to eight years, depending on many factors. The goal of this section is to examine such factors.

Weather Severity

Climate clearly can play an important role relative to the rate and extent of weathering of wood. Grüll et al. (2013) found that a site in the United Kingdom that was known for wet weather gave more rapid degradation of coated pine panels compared to four other European sites. Creemers et al. (2002) carried out matched sets of weathering tests at nine different locations in Europe. They found that the general effects could be simplified by defining a climate index, which was a function of the amount of irradiation, total precipitation, and number of days with more than 0.1 mm of precipitation. Likewise, Bobadilha et al. (2021) found that wood finishes failed more quickly in Mississippi than in Wisconsin, which they attributed to greater decay. Davis et al. (2022) carried out matched weathering tests of coated wood in a warm-summer Mediterranean location and in a semi-arid climate of eastern Oregon in the US. Fungal growth was much more prominent in the wetter site. Dawson et al. (2005) compared weathering of matched pine wood specimens in Germany and New Zealand. As in the previous example, it was found that mold growth played a major role in the degradation of the coated wood specimens. It was found that a higher amount of treatment to suppress mold was required at the New Zealand site, which was the wetter site. Bratasz et al. (2012) compiled maps based on projected climate change, in future years, to be able to estimate the risks to painted wood items.

Wood Selection

Wood species

Several studies have revealed differences in coating performance on different species of wood. Some key findings are highlighted in Table 6.

Table 6. Effects of Wood Species on the Performance of Outdoor Finishes

Notably, certain species such as teak are known to have greater natural resistance to decay (Lestari et al. 2020). However, color change is a priority for many users, who are not pleased in cases where an expensive, reddish-colored wood tends to become gray during exposure to weather (Williams et al. 1996). Feist (2002) provides a broad discussion of how not only the species of wood but also differences between the wood of individual trees can affect the performance of coatings. For example, it has been found that wood having a large proportion of latewood can give rise to adhesion failure of the coating after extensive weathering. The cited article provides a table with ratings of different wood species with respect to paint-holding ability for oil-based and latex paints.

Wood’s inherent antimicrobial performance

Some wood species are inherently more resistant to biological decay. This characteristic often has been attributed to the presence and levels of wood extractives (Kirker et al. 2013). The cited work showed that after extraction of such compounds from the tested wood species, the rates of attack by fungi and termites were higher in nearly all cases. Similar results had been obtained earlier by Taylor et al. (2006), but the correlations were low. Further confirmation of these effects comes from studies in which the extracts from resistant wood species were used to treat non-resistant species, such as pine. Thus, Syofuna et al. (2012) found that certain wood extracts could protect pine wood against termites. Sablík et al. (2016) found related effects when extracting the heartwood of black locust and then impregnating non-resistant beech wood, which then showed resistance against fungal decay.

Wood density

Sjökvist et al. (2019) reported correlations between the density of Norway spruce wood and the tendency of the coated wood to develop cracks during three years of outdoor exposure. The lower-density specimens exhibited higher moisture contents. However, the higher-density specimens showed a higher number of cracks. Follow-up work showed the unexpected effect of higher water uptake into coated high-density wood, compared to coated low-density wood (Sjökvist et al. 2020). Williams et al. (2000) noted that high-density woods tend to swell and shrink to a greater extent than lower-density woods, and that this tendency can lead to a higher rate of checking. It makes sense that checking will hurt the performance of coatings. Strikingly different effects were found in a study of surface-densified wood (Cheng et al. 2024). Such densified surfaces showed superior paint film adhesion, hardness, and resistance to impacts, and these were applied to a range of tropical hardwoods.

Grain coarseness

Better performance has been reported for wood coatings on fine-grained woods. Thus, de Windt et al. (2014) found the lowest degradation rate of coatings on fine-grained wood. The coatings ranged from clear stains to opaque stains. There appears to be a need for more research in this area, not only to confirm the reported observations, but also to seek a mechanistic explanation.

Heat-treatment of wood

The heat-treatment of wood can be regarded as an additional option related to the kiln drying of wood, as illustrated in Fig. 3. Kiln drying, meaning the heating in air for many hours or a few days at about 50 to 90 °C for most hardwoods and above 100 °C for softwoods, is applied to most commercial lumber as a means to stabilize the dimensions of the wood. Due to increased danger of fire, as well as to minimize undesired chemical changes, such as oxidation, technologists use various ways to exclude oxygen when using higher temperatures, often in the range 180 to 220 °C, to heat-treat the wood. Details and options for the procedures and conditions of heat-treatment of wood have been reviewed (Esteves and Perera 2009; Cao et al. 2022; Hubbe and Laleicke 2025).

Fig. 3. Simplified comparison of conditions typically used in ambient drying, kiln drying, and heat treatment

The effects of high-temperature treatment already were considered in a companion article (Hubbe and Laleicke 2025), where the emphasis was on adhesion between the coating and the wood. Heat-treated wood is also of importance in the present context, since it is a known strategy to increase resistance to decay. Here the emphasis is on whether the combination of heat treatment, followed by coating of the heat-treated wood, renders the surface more durable in a practical sense. The highlights listed in Table 7 are focused on such issues.

Table 7. Overall Effects of Heat Treatment of Wood, Followed by Coating, Relative to its Durability

Mechanical Processing of Wood for Adhesion of Finish

There is much evidence that the performance of wood coatings can be improved by appropriate preparation of wood surfaces. Aspects to be considered here include the freshness of the wood surface (i.e. how recently the surface has been machined or sanded), the planing of wood surfaces before coating, and aspects of sanding prior to coating application.

The importance of freshness of the wood surface

Articles cited by Hubbe and Laleicke (2025) showed that fresh removal of the outermost layer of wood is a reliable way to achieve improved adhesion of coatings on wood. Wood extractives are expected to gradually migrate to exposed surfaces of wood over time. The migration is motivated by a minimization of surface free energy when a surface becomes covered by nonpolar compounds. Such compounds, which include fatty acids and resin acids, can be expected to inhibit wettability of the surface by some coatings, especially water-borne formulations. In addition, the monomeric compounds, due to their lack of covalent bonding to the surrounding material, will not be able to provide firm anchoring of the coating. In principle, such problems can be minimized if the outermost wood is removed shortly before coatings are applied.

Adverse effects of weathering of bare wood on subsequent coating performance

Severely adverse effects on coating performance have been reported in cases where the uncoated wood surfaces have already been exposed to outdoor weather (Williams et al. 1987a,b; Williams and Feist 1993, 1994, 2001). To some extent, such problems can be overcome by selecting a specialized finish formulation, such as with 10% linseed oil and 11% acrylic resin (Williams et al. 1999).

Some challenges in the application of coatings on top of weathered wood are highlighted in Fig. 4. As shown by articles cited in an earlier review article (Kropat et al. 2020), one of the major changes resulting from exposure to weather, i.e. combinations of ultraviolet (UV) light exposure and periods of rainfall, is a loss of lignin from the outermost 1 mm or so of the wood. On the one hand, the loss of lignin results in a relatively loose layer of cellulose fibers, which might be regarded as a weak layer. However, the cited research suggests also that there is an inherent incompatibility between the generally hydrophilic nature of cellulose and hemicellulose (after the loss of lignin) and the more hydrophobic nature of typical resins used in wood coating formulations. A study by Kanbayashi et al. (2023), based on Raman spectrometry, showed that lignin degradation can take place even below both penetrating and film-forming coatings. As outlined in another earlier review article (Hubbe and Laleicke 2025), the strength of adhesion between two materials, depending on various process details, is often governed by how similar the two materials are in terms of cohesive energy density, polarity, and capacity to form hydrogen bonds. Further challenges, from the standpoint of establishing lasting adhesion to a coating layer, may be associated with the tendency of lignin-free cellulose fibers to swell a lot when then imbibe water and then to shrink a lot when dried (Hubbe et al. 2024). Depending on the flexibility of the dried and cured coating, such dimensional changes can be expected to induce strong shearing forces at the interface. The presence of lignin, when the fibers are within a sound wood structure, tends to restrain such dimensional changes. Since the lignin is the most hydrophobic of the three main chemical components of wood, and thereby the most similar in terms of wetting properties to the resins used in coating formulations, its presence seems to be important when the goal is to achieve strong adhesion of a coating.

Fig. 4. Idealized illustration of weathered wood surface and its ineffective adhesion to a coating

Figure 5 illustrates the role of fresh mechanical treatment of a surface, as in the case of planing or sanding. The mechanical action appears to have three main objectives, the first of which is to achieve a specified level of smoothness. The second objective, which is sometimes overlooked, is that there ought to be an optimized level of roughness of the surface in order to be able to interact in a three-dimensional manner with the coating layer. Third, the fresh mechanical action can remove any accumulation of waxy substances, which otherwise could impede spreading of the adhesive and development of effective bonding with such wood components as lignin, cellulose, and hemicellulose.

Fig. 5. Schematic description of mechanisms by which fresh sanding can provide stronger adhesion of coatings to wood

As illustrated in Fig. 6, the planing of wood employs a blade to improve the smoothness of a sawn lumber piece. Studies by de Moura and Hernández (2005) showed that improved adhesion of wood coatings could be achieved by peripheral knife planing before high-solids polyurethane coating. The wood surface after planing was described as “undamaged”. Planing to achieve a fresh surface is commonly done during the assembly of Cross-Laminated-Timber, a structural wood panel. Some adhesive manufacturers require a timely planing step prior to the application of the primer (Miyamoto 2024). Cool and Hernández (2016) compared oblique cutting, face milling, and helical planing as alternative preparations of spruce wood immediately before application of exterior acrylic water-based coating. Helical planing achieved superior results, including better adhesion of the coating and resistance to weathering. The worst results were with the face milling, which gave an uneven coated surface and poor adhesion. Follow-up work showed best results with peripheral planing of red oak wood at a rake angle of 25 degrees (Ugulina and Hernández 2017). The authors attributed the superior adhesion of the wood coating to the fibrillation of the surface in the course of the planing. Altun and Esmer (2017) likewise achieved good adhesion of varnish to iroko wood that had been planed.

Fig. 6. Simplified diagram of a rotary planing operation

Though a state-of-the-art planing treatment carried out recently before application of coating might be favorable for the overall goals of a project, it is important to note that a roughly sawn surface may provide superior adhesion of a coating. Thus, Nussbaum et al. (1998) reported superior adhesion of alkyd and linseed oil finishes onto sawn and rough surfaces.

Sanding

It is notable that de Moura and Hernández (2005), even when showing positive effects on coating adhesion after planing of the wood surface, found even better results following sanding. When using sanding to prepare a wood surface for application of a coating, the general recommendation is to start with a level of grit suitable to remove the largest features of roughness, such as grooves created by previous sawing or planing actions (Allen 1984; Flexner 1994). Detailed measurements of the features left behind by sawing have been quantified by Singh and Dawson (2006) and Singh et al. (2007). Subsequently, the sanding is repeated with successively finer (higher number) sandpaper but not continuing past what is needed. Thus, Allen (1984) recommends starting with 100 grit and proceeding up to 150 grit for routine house painting or 220 grit for varnish applications. Table 8 lists articles dealing with the role of sanding is preparing durable external wood coatings.

Figure 7 illustrates the point made by Landry and Blanchet (2012a) regarding optimization of sanding practices in the case of typical outdoor wood coating projects. As shown, it has been recommended that a medium coarseness sandpaper such as 160 grit may be a good choice for such projects as the final sanding before application of a primer coat. When such sandpaper is replaced with sufficient frequency so that it will be sharp and not filled with wood particles, it can be expected to provide enough fibrillation and microscopic tearing of the wood surface to provide strong adhesion at the wood-coating interface.

Table 8. Sanding Used as a Means to Improve the Performance of Wood Coatings for Outdoor Applications

Fig. 7. Schematic illustration of the importance of selecting a sufficiently coarse sand paper and replacing it often enough as a means to achieve good long-term coating adhesion

One of the commonly repeated points of advice regarding the sanding of wood is to use fresh sandpaper that is replaced with suitable frequency (Flexner 1994). In addition, as noted by Allen (1984), it can be helpful to periodically slap the paper on a hard surface to dislodge wood particles and expose the sharp mineral grains. Such practices are consistent with the idea that loose material left on the wood surface might not be helpful for developing strong adhesion to a coating. Conversely, a sharp sandpaper sheet may be helpful in creating fibrillation or minor tearing of the wood surface in such a way as to achieve better mechanical interlocking at a suitable length scale, such that the coating becomes well attached, but the surface of the coating does not become rough.

Chemical Treatment of Wood for Finish Durability

Though it has not become a common practice, there has been some research related to potential chemical treatments of wood surfaces in advance of coating application. These can range from bleaching to extraction of the wood surfaces, to plasma treatments, and finally to chemical derivatization of the wood surface.

Bleaching

Bleaching of wood surfaces provides a means to achieve a lighter appearance of wood, when that is desired. Atar et al. (2004) compared a series of different bleaching treatments, all based on hydrogen peroxide, for beech wood in preparation for varnish application. All of the treatments decreased the hardness of the unfinished wood surface, but after varnish application, the initial hardness had been restored in all cases. Budakçı (2006) showed that bleaching treatment can be effective for restoration of wood’s color after weathering of clear-coated wood.

Going another step beyond conventional bleaching treatments, Dawson et al. (2008a,b) explored the feasibility of intentionally removing lignin from the outermost 2 or 3 mm of the wood surface before application of a clear coat. The idea was to preemptively remove the component of wood that is susceptible to photodecomposition in the presence of ultraviolet light and thereby stabilize the system. The desired delignification was achieved by means of oxidation with peracetic acid. The resulting surface was judged to still have sufficient strength to work well with the coating. It is worth noting, however, that surface delignification also can be achieved by natural or conventional weathering, the results of which are often unfavorable to the adhesion of wood coatings (Kleive 1986; Hubbe and Laleicke 2025).

Extraction of wood

Ghofrani et al. (2016) showed that stronger adhesion of varnish to wood could be achieved by removal of extractives. Alder and ironwood were compared, and the extractive media were either hot water or ethanol. The extracted surfaces showed superior wettability to the two-part waterborne urethane-alkyd varnish formulations, as well as showing higher pull-off strength. The authors attributed the improvements to avoidance of the blocking action of the wood extractives. The extractives would have prevented direct contact between the coating and the polymeric components of the wood. Wu et al. (2020) used nanoindentation to show that a waterborne formulation achieved higher hardness after extractive removal from a wood surface. The benefit was attributed to the better wettability of the surface by aqueous media.

Plasma pretreatment of wood

As discussed in recent review articles (Klébert et al. 2022; Hubbe and Laleicke 2025), treatment of a wood surface with plasma, especially the so-called corona treatment that involves relatively low energy plasma in air, can be used as a way to increase adhesion, especially in the case of water-borne coatings. As illustrated in Fig. 8, some of the potential beneficial effects of corona treatment, possibly in preparation for the coating of wood, can include the cleaning of waxy substances from the surface, the etching of the surface on a microscopic level, and oxidation of the surface, leading to good wettability by aqueous formulations (Klébert et al. 2022; Zigon et al. 2022). Here the emphasis is on developing resistance to outdoor exposure. Reinprecht and Somsák (2015) and Reinprecht et al. (2020) showed that plasma treatment aided the stability of clear acrylic and alkyd coatings on spruce specimens that were exposed to artificial weathering. Gholamiyan et al. (2022) compared plasma treatments with air, nitrogen, and carbon dioxide media on the performance of waterborne and solvent-borne coatings on fir. They found increased weather resistance of both waterborne and solvent-borne coatings. Even though the plasma treatment rendered the wood surface more hydrophilic, the final result achieved greater hydrophobic character of the coated surface. Kettner et al. (2020) pointed out that resinous woods, including pine, are often preferred for outdoor applications, in recognition that the hydrophobic nature of wood resins can help resist water, but that the same resins can hurt the performance of coatings. They were able to overcome the adverse effects by a specialized plasma treatment involving chemical vapor deposition. Blue-stain fungal infestation was effectively suppressed by the treatment before application of a glaze or lacquer coating.

Fig. 8. Reported effects of corona treatment, a relatively inexpensive cold plasma treatment using air, for preparation of a wood surface to receive a coating

Czarniak et al. (2022) found better wettability of common wood finishes after corona treatment; however, there was no corresponding improvement in adhesion of the coating layer. Thus, it makes sense in practical cases to determine whether or not plasma treatment is either needed or beneficial, depending on various details.

Acetylation of wood

By treatment of the wood surface with acetic anhydride, some of the -OH groups that are prevalent especially within the hemicellulose and cellulose components can be converted to their acetylated forms. Such an operation is illustrated schematically in Fig. 9. This kind of treatment can be interesting from a wood protection standpoint, since acetylated wood is much more resistant to wetting and swelling in water (Rowell and Bongers 2015). The cited authors explained that the improved dimensional stability of the wood, relative to changing moisture conditions, reduces the stress at the coating-wood interface, such that less there will be less tendency for peeling of the coating over time. Studies have shown that acetylated wood is more resistant to weathering (Beckers et al. 1998; Fodor et al. 2022). Beckers et al. (1998) showed furthermore that acetylated pine wood still adhered well to a variety of different wood primers, including waterborne and solvent-borne. Fodor et al. (2022) reported decreased absorbance of pigmented stains into acetylated hornbeam wood, though the acetylated wood was more durable and dimensionally stable under outdoor conditions. Although acetylation has been shown to increase the service life of exterior wood, as in the case of wood siding on houses, it does not prevent color change due to weathering (Sandak et al. 2021). The cited authors showed that so-called hybrid processing, using a range of commercially available treatments, all of which were coatings. Nagarajappa et al. (2020) showed that the photostability of acetylated wood could be improved by adding an ultraviolet light absorber, ZnO, to the coating formulation.

Fig. 9. Surface acetylation of wood, using acetic anhydride

Wood Impregnation and Wood Finish Performance

Resin impregnation

Wood impregnation has become a popular way to improve its weathering characteristics, especially with respect to attack by fungi and termites. As illustrated in Fig. 10, such treatment can be achieved by first immersing the lumber pieces in a solution or suspension of the material to be impregnated, then applying vacuum, and then releasing the vacuum. The evacuated pores of the wood then suck the impregnating fluid into the pores of the wood, including vessels and fiber lumens. Subsequently, the suspending medium is expected to evaporate.

Fig. 10. Schematic diagram of steps in a process of impregnating wood by use of application and release of vacuum

Table 9 highlights some work that has been carried out to find the impact of such impregnation on the performance of coatings. The most commonly studied impregnants have been copper-based antimicrobial treatments and formaldehyde resin formulations. In a majority of cases, the impregnation resulted in better resistance to weathering.

Table 9. Effects of Wood Impregnation on the Performances of Exterior Wood Finishes

Even in the absence of biocidal treatment, it has been shown that various clear and tinted coating formulations can provide resistance against mildew for 12 months or so, followed by a decline (Morrell et al. 2001). After 21 or 39 months of weathering, the coating surfaces on western red cedar were generally found to be hydrophilic and in poor condition.

Primer Coats and Finish Performance

Why apply a primer?

The most important aim of applying a primer coat to a wood surface is to provide strong adhesion to the wood and thereby achieve reliable anchoring of subsequent coating layers. The practice of employing a primer layer for this purpose is consistent with an expectation that the wood-coating interface is likely to be a point of failure, including eventual peeling of coating from the wood (Knaebe et al. 1996; Williams et al. 1996). Another part of this general expectation is that adhesion between successive layers of coating is likely to be relatively strong, especially if only hours or days have separated application of the different layers. A further role of the primer coat, especially when covering resinous wood species, is to act as a sealant against permeation of wood extractives into the final layers of coating (Cassens and Feist 1986). As was illustrated in Fig. 2, excessive migration of resin from freshly applied coating into the pores of the wood can contribute to chalking; it follows that one of the roles of the primer, in addition to other intermediate coating layers, is to reduce such undesired depletion of binder from the top coating layer when it is still wet. According to the USDA Forest Products Laboratory (1972), “The fist, or prime coat of paint, is the most important coat to be applied to wood.” Despite this assertion, the literature search carried out in preparation of this article found that relatively little research has been carried out relative to the performance of primers.

Figure 11 illustrates the concept of using a primer. As shown, the low viscosity and relatively low solids content of a primer allows it to penetrate deeply into vessel openings, lumens, and fiber-level tear-out defects near to the wood’s surface, thus achieving good chemical and mechanical bonding. Achieving such a beachhead of good adhesion can be very important, especially if the wood-finish interface later becomes the critical site for initiation of peeling.

Fig. 11. Schematic diagram contrasting the deep penetration to be achieved by an idealized primer coat, vs. a typical film-forming finish coating formulation

Typical primer formulations

Consistent with the goal of penetrating well into the pores of the wood surface, primers are generally formulated with a relatively high content of liquid medium (Hubbe and Laleicke 2025). The USDA Forest Products Laboratory (1972) has recommended usage of an alkyd oil-based formulation, with inclusion of TiO₂ particles as a preferred primer choice. Liu et al. (2021) suggested using a polyurethane sealing primer on poplar wood. However, the main reported benefit was a higher gloss, not resistance to weathering.

In cases where resistance to discoloration of coatings by wood resins is a priority, Coniglio (2023a,b) have recommended using a water-borne primer, followed by a solvent-borne subsequent coating layer. This combination was reported to be very effective in protection against the bleeding of extractives from knots. Pánek et al. (2017) advocated priming oak wood surfaces with a transparent, hydrophobic coating to prolong the service resistance of coatings the undesired bleeding of tannins.

Considering the widespread agreement among experts that primer application should not be skipped, it seems remarkable how few times the formulation of primer coatings was found in the literature search for this article. It follows that the formulation of primer coatings, relative to such factors as coating adhesion, wood resin migration, and resistance to weathering effects, can be regarded as priority topics for future research.

Water-repellent preservative

Several articles have described the application of formulations called water-repellent preservatives, which appear to play a role analogous to that of a primer, or maybe as an impregnation treatment preceding usage of a primer coating. Highlights from such publications are provided in Table 10.

It is notable that Williams et al. (1996) disclosed the usage of wax, which appears to have been the main hydrophobic ingredient in the formulations being recommended in a series of articles and publications coming from employees of the USDA Forest Products Laboratory. However, the application of wax prior to coating prompts the following question, which might be considered in future research: Are there circumstances under which the wax itself may play a role as a weak boundary layer, thus increasing the likelihood of future peeling of a coating layer at its interface with the wax or within the wax layer? That question does not appear to have received an answer in the published literature.

Table 10. Effects of a Water-repellent Preservative, as a Primer Coating, on the Performances of Exterior Wood Finishes

Primer coats acting alone, vs. weathering effects

Though primer coats are not usually envisioned as acting by themselves, without at least one subsequent coating layer, there are some special circumstances under which this may happen. For instance, there might be a change of weather that delays the application of the intended subsequent coats. Ahola (1995) has reported that under such conditions, application of a pigmented stain (which essentially would be serving as a primer coat) made it possible to minimize adverse effects of leaving the wood exposed.

Table 11. Effects of the Choice of Finish Type on Resistance to Weathering

Coating Type

When considering different options to achieve resistance to weathering, one of the important questions is whether the type of coating, i.e. the main resin ingredients, makes a difference in terms of its effectiveness and service lifetime. Table 11 highlights studies that shed light on such issues. A notable point about this collection of cited work is that there have been relatively few reported studies comparing completely different wood coating types under matched weathering conditions. In addition, none of these cited reports gave persuasive evidence that any particular type of wood coating ought to be completely avoided for outdoor applications. On the other hand, to help answer questions regarding weathering issues, it would make sense to carry out a systematic study involving a large number of coating types, some involving use of a primer coat, all on the same wood and with the same weathering conditions. An interesting research question to consider is whether or not a wide range of commercially available formulations intended for outdoor use all show comparable resistance to color change, cracking of the coating, checking of the wood, or peeling when subjected to the same weathering conditions.

Finish Coat Application Factors and Durability

Another kind of question to consider is whether the results of coating performance, especially in exterior applications, depend on such factors as the application method, the number of coats, the total coat weight, the method of application, the time passage between applications of successive coats, or the weather conditions (e.g. humidity or rain) during or immediately before the application. Here, attention will be paid to the finish coats, i.e. the coating layers that follow the primer coat.

Application methods, brushing, rolling, wiping, and spraying

In principle, coating formulations can be applied to wood surfaces by brushing, rolling, wiping, dipping, and spraying (Williams et al. 1996; Landry and Blanchet 2012a). These four systems are illustrated in Fig. 12.

Fig. 12. Common systems for application of wood coating formulations

Of these, brush painting is the most common for coating of wood in external applications. Flexner (1994) provides a detailed description of the most popular type of paintbrush, which typically has a chisel-shaped tip and taper bristles that are flagged into multiple filaments near their tips. The flagging provides some extra capacity to hold wet coating formulation. In the case of roller application, which is widely used for applying paint to gypsum board (drywall), the fineness of the pattern has been shown to depend on the nap (meaning the size of the filaments in the felt-like material). In addition, there has been some work on the relationship between spattering of paint during roller application and the rheological properties of the formulation (Chaudhary et al. 2020). However, the literature search carried out for this article did not discover any scientific study to compare different application methods with each other with respect to outcomes such as the quality and service life of the resulting coating.

Number of coats

Several studies have considered issues related to the number of coats that should be applied to wood for exterior usage. Key findings or recommendations of those studies are highlighted in Table 12. In terms of effects on weathering, the most pertinent findings were from the 1972 USDA Forest Products Laboratory publication, since it basically predicted an increased service life of about 7 years due to application of a second finish coat. These findings are reinforced by work of Grüll et al. (2014). The results of Hysek et al. (2018) provide circumstantial evidence that a higher number of coatings has the potential to achieve a more uniform final effect.

Table 12. Findings from Studies Considering the Number of Coating Layers for External Wood Applications

Figure 13 illustrates some reasons to expect that multiple layers of a finish coating ought to have a more durable effect than just one. If it is assumed that each coating will have defects, such as gaps or thin spots, the next layer can be expected to fill in such defects. If such defects are assumed to be random, then the application of multiple layers decreases the probability that inadequately covered areas will remain. Finally, multiple coatings allows for a greater overall thickness, while avoiding such problems as sagging of wet coating on vertical surfaces, which might be expected if excessive amounts are used in individual layers.

Fig. 13. Illustration of how the usage of multiple coatings may be able to compensate for un-evenness of individual coating layers, leading to a thicker and more durable combined coating

Net coating weight

Grüll et al. (2014) found that higher total film thickness led to greater resistance to the effects of weathering. Liu and Xu (2022) found that there was an optimum layer thickness when implementing a three-layer system in which a sealing primer was covered by a default primer and then a polyurethane topcoat. The sealing primer layer thickness did not show any significant effects. Best performance, in terms of adhesion and gloss were achieved with wet layer thicknesses of 60 g/m² for the sealing primer, 100 g/m² for the waterborne primer, and 120 g/m² for the self-made topcoat formulation.

Given the finding that coating performance often has been correlated with increasing total coat weight, it is worth considering whether it is feasible to increase the thickness of individual layers. Allen (1984) states that excessive layer thickness is likely to give rise to wrinkling. Such an effect might be expected on vertical surfaces in the case of coatings having a tendency to form an outer skin in the early stages of drying due to evaporation from the surface. A high thickness would imply a large amount of wet, low-viscosity material persisting long enough below the surface to allow the outer skin layer to sag, possibly in an uneven manner, giving rise to wrinkles. Cox (2003) recommended an ideal layer thickness of 4 wet milli-inches (equal to a total of about 0.1 mm) for a primer layer and 3 wet milli-inches (~ 0.075 mm) for a topcoat.

Time between coating applications

In principle, it is expected that there should be an optimum window of time during which to apply a second or layer coating layer to wood. Cox (2003) pointed out that sufficient time must have passed in order to allow the previous layer to have dried. The USDA Forest Product Laboratory (1972) and Cassens and Feist (1986) recommend applying each layer within two weeks of application of the preceding layer. As illustrated in Fig. 14, presumably with the further passage of time, the polymer segments within a coating layer become increasingly difficult to mobilize, thus limiting the amount of mixing of polymer segments at the interface between layers when applying the next coating.

Fig. 14. Schematic illustration of an effect of time between application of successive coating layers on the development of polymer segment mixing at the interface

Sanding between coats

It makes logical sense that abrading a layer of coating with a suitable fine sandpaper ought to increase its adhesion to the next layer, due to mechanical interlocking, but the questions to consider is whether or not such improvements are needed and whether the effects are large enough to be worth the effort. As already was shown in Table 12, Flexner (1994) recommends sanding between layers of coating when there is a need to improve adhesion between them. Liu et al. (2021) reported improved bonding of a nitrocellulose transparent coating to a nitrocellulose transparent primer on poplar wood. However, the application was antique furniture, not exterior wood. It would be interesting to test this approach for systems that are subsequently exposed to weathering.

Weather conditions during application

Though it is reasonable to discontinue exterior painting well before the onset of a rainstorm, the weather circumstance most often mentioned by experts is almost the opposite – the warming of previously cold wood by sunlight (Ulfvarson and Pattyran 1972). A likely cause of this problem is illustrated in Fig. 15.

Fig. 15. Schematic illustration of what can happen when a fresh coating is applied to cool, damp wood not long before the rising of the sun, thus allowing a skin to form on the surface of the wet coating layer, followed by buildup of vapor pressure from the evaporation of water in the wood

When cold wood is coated, and when the sun subsequently is able to heat the surface before drying of the coating, it is likely that substantial liquid (water or solvent) may still be present underneath a skin of cured resin. Such a situation can give rise to blisters (USDA Forest Product Laboratory 1972; Williams et al. 1996; Cox 2003).

Moisture Control and Finish Durability

Water, together with ultraviolet light, can be regarded as a key enemy to the preservation of exterior wood. In particular, multiple episodes of wetting and drying will give rise to periodic vertical cracks, i.e. checking (Feist 1982b; Kropat et al. 2020). The wood swells each time that it is wetted, and it shrinks each time that it is dried. Steep moisture gradients can arise, for instance when wet wood is exposed to strong sunlight. It follows that the wood will experience shear stresses periodically during such moisture cycling (Flexner 2005). These shear stresses can result in check formation.

To combat the effects just described, an effective wood finish for exterior use needs to do two things.

• First it needs to block liquid water (from rain, dew, etc.) from entering the pores of the wood.
• Second, it needs to have sufficient vapor permeability to allow any water that might be trapped below the finish to gradually diffuse out. If not, there is a danger that the wood may decay (van Meel et al. 2011).

Hydrophobic character of finish

During rainstorms, as well as morning dew, coated wood surfaces are periodically exposed to liquid water. Many studies have been carried out to evaluate different additives that might act as hydrophobic agents, often with the goal of decreasing the amount of water that might permeate through the coating. Figure 16 highlights three classes of hydrophobic agents that can be components of various exterior coating formulations for wood. The main findings of such studies are highlighted in Table 13. As shown, the studied treatments have been highly diverse. Though the concept of using hydrophobic additives in wood coatings appears to be widely shared, no particular type of treatment has achieved a large proportion of research attention.

Though research into hydrophobic additives has shown that the contact angle of water on wood coatings can be increased, it is worth keeping in mind that the initial hydrophobic charter of such a surface can be expected to change over time due to the gradual oxidation of coating layers, the build-up of dirt, and any biological growth, such as mildew. Though it is possible, in the lab, to prepare surfaces so hydrophobic that drops of water just roll off of them (Huang et al. 2022), it is realistic to expect that coating wood surfaces exposed to rain will soon become completely wetted by water.

Even if the outside surface of a coating layer becomes completely wetted by water, especially with continued rainfall, that does not necessarily mean that the hydrophobic nature of a wood coating does not play a role in keeping the underlying wood dry. Suppose, for instance, that the coating layer has capillary pores within it; a hydrophobic nature of the walls of such pores will be expected to resist penetration by water (Lucas 1918; Washburn 1921). Such a mechanism can become important in cases where there are pinhole-like features in a coating layer.

Table 13. Studies Evaluating the Addition of Hydrophobic Components to Coating Formulations for External Applications

The next topic to consider is factors affecting the rate of diffusion of water monomers (i.e. water vapor) through the coating itself, regardless of whether or not the external surface has a low contact angle with liquid water.

Fig. 16. Three classes of hydrophobic agents that may be present in coating formulations

Moisture barrier issues

Table 14 highlights key findings from studies that have considered the rate of permeation of water vapor and other monomeric substances through layers of wood coatings. Of particular note is the point that waterboard coatings, which involve the coalescence of polymer segments on adjacent latex particles during evaporative drying and curing, are known for providing greater breathability, meaning that their water vapor transmission rate is higher (Allen 1984). Another key point is the finding that defects such as cracks in a coating layer can be expected to have a disproportionately large effect on the overall protective ability of a coating (Williams et al. 1996). For example, interfacial incompatibility between a kind of particle and a kind of resin within a coating layer has potential to yield air gaps, which can serve as a conduit for water vapor transport (Donkers et al. 2013).

Table 14. Studies Considering Rates of Diffusion and Factors Affecting Rates of Diffusion of Water Vapor and Other Monomers through Wood Coatings

Ahola (1991) compared nine different joinery paints with respect to their rates of water vapor and liquid water transport on pine and spruce woods. Tests were done in outdoor conditions for a year. One of the biggest contrasts was the much slower flux of water into wood when using film-forming coatings (typically about 1 to 1.5 kg/m²s) rather than a stain (about 5 kg/m²s). Surprisingly, three years of weathering did not have a large effect on the transmission rate, with a few exceptions. Follow-up work showed that the absolute rate of water permeation through the finish did not depend on the underlying wood species (Ahola et al. 1999). Williams et al. (1996) mention a commercial “vapor retarder” product having the purpose of slowing down the diffusion of water into wood through the coating layer. However, the note was made that in many cases air leaks through cracks or other defects in a coating may have a more important effect than the rate of diffusion through unblemished parts. Allen (1984) noted that oil-based primers tend to be resistant to vapor diffusion, whereas water-based primers can be generally described as breathable. Gezici-Koç et al. (2019) found a correlation between the amount of water present in a coating, on the wood surface, and its rate of diffusional transport of water vapor. Similar results were obtained for two alkyd coatings (one solvent-borne and one waterborne, and a water-borne acrylic formulation).

Excessive resistance to water vapor diffusion can be a problem in cases where the wood tends to retain too much moisture after rainfalls and a coating slows subsequent transport and evaporation of that moisture (Bronski and Ruggiero 2000). The cited authors pointed out that surface-sealing barrier coats may tend to entrap moisture within a wall system.

Rapp and Peek (1999) likewise blamed a varnish layer for long-lasting periods of high moisture content during natural weathering of spruce, oak, and Douglass fir specimens. On the other hand, as noted by van Meel et al. (2011), a coating also can reduce the amount of moisture taken up by the wood during rain events, leading to net positive results in typical cases.

Binder Attributes and Durability

The question considered here is whether certain attributes of the binder, i.e., the softness or stretchability of the polymer used in different wood coating formulations, has been found to correlate with its performance in external wood coating applications. Thus, de Meijer and Nienhuis (2009) considered the susceptibility of different coating formulation to develop internal stresses and compared those findings to cracking and flaking results.

It was found that high levels of stress were developed when there were large differences in swelling between the wood and the coating. Results were compared with accelerated and natural weathering test results, and a good correlation was found. Best results were found with those primers (especially solvent-borne) having less swelling tendency and therefore less stress development at the interface.

According to Baumstark and Tiarks (2002), an ideal resin for exterior wood coatings ought to be non-tacky, hard, resistant to blocking, but also very flexible, such that it will be resistant to cracking. They found that those criteria were best met, in the case of acrylic binder systems, by using multi-phase acrylic emulsions. A combination of a soft hydrophobic monomer (2-ethyl hexyl acrylate) and methyl methacrylate gave the best resistance to weathering, including the least uptake of water by the wood. Rather than try to achieve a uniform or random copolymer, it was found that particles with soft and hard phases performed best. A particle size of about 100 nm was recommended, such as to allow good penetration and distribution.

Silicones for durability

Silane and silicate-type coatings appear to offer a wide range of possibilities for such purposes as cross-linking and the development of hydrophobicity in wood coatings. Highlights from related studies are provided in Table 15. As indicated by the table contents, the properties provided by these treatments were quite diverse, such that time and effort will need to be devoted to formulation of the best ratios of additives in various cases. Alkyltrialkoxysilanes and related chemicals were effective both as crosslinking agents and as hydrophobizing additives (Husić et al. 2023). Systems based on precipitation of water glass (a highly alkaline solution of silicate) tended to be brittle, but suitable as a pre-coating treatment; film-forming additives can be employed to overcome the brittleness (Tshabalala et al. 2011).

Table 15. Studies of Effects of Different Formulations of Silane-type or Silicate-type Components in Exterior Wood Coatings

UV-cured finishes and durability

Decker (1996) appears to have been the first to describe the development of a UV-cured coating for exterior application of wood. A large increase in weatherability of the wood specimens was observed in comparison to uncoated wood. No pigmentation or absorbers for UV light (beyond the photosensitizer for curing of the resin itself) was employed. Gurleyen (2021) improved weathering performance by application of UV-cured varnishes for flooring applications. Again, no additional UV-absorbing materials were employed. Weichelt et al. (2010) developed a UV-cured coating for outdoor wood application. This was done with the addition of nano-sized ZnO particles to protect against ultraviolet light. Vardanyan et al. (2015) showed that adding cellulose nanocrystals to a UV-curing polyurethane acrylate transparent coating formulation decreased color change during accelerated weathering. Irmouli et al. (2012) found that adding UV absorbers having different absorbance spectra compared to the photoinitiator did not provide any improvement in resistance to artificial weathering of UV-cured clear finishes on oak and spruce woods. Dixit et al. (2021) did not consider weathering effects, but they formulated a UV-cured wood coating system to have antimicrobial properties. Because UV-curing coating formulations already have to contain an initiator that absorbs UV light, there is reason to suspect that there might be a significant contribution of that additive to later protecting the coating and the wood during outdoor exposure. However, there does not appear to have been a study addressing such a contribution to the performance of UV-cured coatings.

Reinforcing Particles and Durability

Many coating formulations that are intended for external usage on wood are filled with particles. Often it is possible to classify such particles based on certain roles that they play with respect to the performance of the coating product. For instance, the word “filler” can be used in cases where a prime consideration is to decrease the overall cost of the formulation. But in addition, it is possible to utilize solid additives for the purpose of enhancing the hardness or other strength properties, and such components can be regarded as reinforcing particles. Another category is particles that have a propensity to impede diffusion of such items as water vapor or air. Finally, it makes sense to consider the extent to which special advantages might be achieved by the addition of particles that interact with light. The refractive index of TiO₂ particles is high enough (2.55 to 2.7) so that there is a large contrast with the typical refractive index of binders (often 1.5 to 1.6), which can give rise to strong scattering of light if the particles are in the diameter range of about 0.2 to 1 µm. Adding such particles to a coating formulation contributes to opacity. When the goal is to achieve high absorption of UV light, one option is to use nano-sized TiO₂ particles. For example, the tendency for particles to contribute to the scattering of visible light, leading to increased opacity, decreases more and more with decreasing diameter below about 200 nm (Jalava 2006). The cited article quantifies how decreases in particle size of TiO₂ particles in the range 237 down to 166 nm tend to shift the interactions with light more and more into the ultraviolet region.

Fillers

According to Flexner (1994) a filler in a wood coating is just a binder with a mineral particle inside. To the extent that such a description is valid, it underscores the idea that some minerals might be regarded as merely occupying space. Yan et al. (2018) used inexpensive talcum powder or calcium carbonate particles and achieved good hardness, adhesion, and impact strength. Relatively inexpensive mineral particles can be obtained, for instance from calcination of rice husks; Guo et al. (2019) found that such material could be added to waterborne acrylic coating recipes. But even in that case, substantial benefits were observed, including increases in tensile strength, elastic modulus, and pencil harness of the coating. Likewise, Meng et al. (2020) added silica particles, but the source of the material was a combination of sol-gel and thiol-ene reactions. The particles were added to a castor-oil-based waterborne acylate formulation. Again, increases in Young’s modulus and hardness of the resulting coatings were observed. Thus, even when considering examples from the literature that would best correspond to the idea of merely filling the coating film with an inexpensive kind of mineral, many researchers have reported reinforcing effects as well. None of the articles cited in this paragraph had anything to report with respect to weathering, however.

Reinforcing particles, including nanocellulose

In principle, when the goal is to increase the strength or elastic modulus of a coating film, three attributes to look for in a particle are its interfacial compatibility with the resin, an elongated or even fibrillar character, and an inherent strength that is sufficiently high. These three attributes, ideally, would allow each of the particles to interact with multiple resin particles, for instance in a typical latex formulation. As an example, Yan et al. (2019) employed glass fiber powder in waterborne coatings at levels ranging from 1 to 7%. The best overall performance was found at a 3% level. A key benefit was in preventing decoloration of a thermochromic ink in the formulation. However, strength benefits were not observed in that study.

Greater success, in terms of strength development, has been reported for nanocellulose, which typically has a generally fibrillar character. For instance, Vardanyan et al. (2014) added cellulose nanocrystals (CNCs) to a UV-cured waterborne coating formulation. Increased scratch resistance, hardness, and adhesion to the wood were observed. Veigel et al. (2014) obtained similar results with the same additives and a waterborne wood coating. Poaty et al. (2014) reported follow-up work in which either alkyl quaternary ammonium bromide or acryloyl chloride was used to render the CNC surfaces more hydrophobic and thereby more compatible with an acrylic resin in the coating formulation. The presence of the compatibilized CNC particles raised the abrasion and scratch resistance by 24 to 38%. Cheng et al. (2016) utilized a certain kind of nanocellulose nanofibers (called TEMPO-oxidized) in a waterborne polyurethane coating. Young’s modulus and hardness were increased. However, the nanocellulose at a 1% addition by mass tended to hurt the adhesion of the coating. Likewise, Kluge et al. (2017) added cellulose nanocrystals and nanofibrillated cellulose (NFC) at the 0.5 and 2% levels to waterborne wood coating formulation. Substantial increases in tensile strength (to breakage) and elastic modulus were found, especially at the 2% level of the NFC. Grüneberger et al. (2014) added NFC to eight commercial acrylic and alkyd polymeric binders; all the resulting films were stiffer, stronger, and less extensible. Pacheco et al. (2021) prepared a composite from SiO₂, TiO₂, and nanocellulose in waterborne varnish and improved the mechanical performance and stability. Shimokawa et al. 2021) showed that adding NFC to waterborne primer formulation resulted in increased breaking stress by a factor of about 1.5. Song et al. (2023) showed that NFC and CNC particles, prepared directly from lignocellulose, improved the hardness, abrasion resistance, and adhesion strength of a waterborne wood coating. Neelambaram et al. (2023) likewise found strength gains upon addition of NFC to a siloxane acrylic latex formulation. Wang et al. (2023) reviewed related work in which nanocellulose has been used as a reinforcement in coatings. It is notable, however, that none of the work mentioned so far in this paragraph involved analysis of the effects of weathering. A rare exception was research reported by Yoo and Youngblood (2017), who studied the effect of adding CNC to tung oil finish formulations. Greater color stability was observed due to the presence of the CNC during weathering of the coated wood. Likewise, Yuan et al. (2021) reported strong reinforcing effects of NFC, in the presence of carbon nitride nanosheets (as a UV absorber), which together improved the weathering resistance of a transparent coating on wood.

Though many of the studies just cited, involving addition of nanocellulose products to wood coating formulations, can seem promising in terms of developing Youngs modulus and strength attributes of the coating films, research attention will be needed with respect to the rheological behavior of such coatings. It is well known that the presence of nanocellulose in aqueous systems can greatly increase the viscosity of the mixtures (Hubbe et al. 2017; Li et al. 2020; Koo et al. 2021).

Tortuosity and effects on vapor diffusion

In principle and in practice, platy particles such as clays are expected to be the most effective in decreasing the permeation of water vapor or other gases through a coating film (Wolf and Streider 1990). This expectation is supported by Nkeuwa et al. (2014a), who considered nanoclay in transparent UV-cured coatings. In particular, multilayer coatings achieved good barrier properties. However, contrary results were observed by Donkers et al. (2013). The latter results raise a suspicion that there may have been poor contact within the coating layer between the resin and the mineral, thus allowing water to migrate through void spaces within the coating. For instance, a failure of the resin to wet the surface of the mineral during formulation and drying may have resulted in gaps between the mineral surfaces and the resin within the dried coatings. Again, none of the studies cited in this paragraph specifically studied weathering effects. Given that mineral particles are relatively inexpensive, as well as the potential benefits of increasing the tortuosity of exterior coating products, it is recommended that such studies be carried out as a priority.

UV Light Absorbers: Mineral

Another key purpose of adding minerals to a coating formulation, especially for outdoor applications, is for protection of both the coating and the wood against adverse effects of ultraviolet light. As noted in an earlier review article (Kropat et al. 2020), the combination of UV light with periodic rainfall is expected to result in degradation and removal of the lignin from the outer layer of unprotected wood.

As illustrated in Fig. 17, one can place UV-absorbers, as used in coating formulations, into three categories. When using high refractive index particles, such as conventional TiO₂ products, both visible and UV light will be impeded from transmission through the coating. When the mineral particles are very small or have a refractive index similar to that of the resins in the coating, then it may be possible to absorb the UV light without affecting the visible appearance. The third category is organic compounds having sufficient conjugation of unsaturated carbon-to-carbon bonds in their molecular structure to allow them to absorb UV light.

Fig. 17. Three classes of UV-absorbing components that can be used in coating formulations

TiO₂ pigment

Due to its high index of refraction (2.55 or 2.7), titanium dioxide has proven itself very effective in the scattering of light (Jalava 2006), making it a good choice for increasing the opacity of coatings. High-quality white paints are usually rich in TiO₂. Another reason to favor TiO₂ in external wood coatings is its strong ability to absorb UV light. In terms of nomenclature, many authors have used the term “nanoparticle” when describing the types of TiO₂ employed in wood coatings. It should be noted, however, that TiO₂ particles in the diameter range of about 200 to 500 nm had become well established in the paint industry long before nanotechnology had emerged as a popular topic. Thus, the term “nano” tends to be absent in much of the older literature. When formulators wish to minimize the scattering of light, which leads to opaque coatings, they need to use even smaller particles, e.g. with diameters less than about 100 nm (Allen et al. 2002). Table 16 highlights studies in which the addition of TiO₂ to exterior wood coating formulations was evaluated. Schaller et al. (2012) have reviewed related literature comparing TiO₂ with other light stabilizers. In addition, Ganguli and Chaudhuri (2021) have reviewed the topic of using nanomaterials, including TiO₂, to minimize biodegradation of the underlying wood.

Table 16. Studies Considering the Effects of Adding Titanium Dioxide Particles to Exterior Wood Coating Formulations

ZnO pigment

After TiO₂, the next most studied UV-absorbing additive for wood coatings has been ZnO. Especially in cases where the goal is not to increase the opacity and hiding power of a paint layer, there may not be a strong need to consider TiO₂, which has a higher refractive index (2.55 or 2.7) in comparison to ZnO (1.54). However, in common with TiO₂, ZnO is photoactive (Thi and Lee 2017), which can raise concerns about undesired generation of high-energy species such as free radicals during service life of the coating, leading to possible breakdown of the coating or the underlying wood. For instance, such effects might possibly be responsible for an observed higher frequency of crack development in some coating systems containing ZnO particles (Allen 1984). However, as in the case of TiO₂, the undesired photocatalytic effects can be reduced by coating the ZnO particles with other materials (Weng et al. 2014; Yin and Casey 2014).

Table 17. Studies Considering the Effects of Adding Zinc Oxide Particles to Exterior Wood Coating Formulations

Other minerals

Table 18 lists key findings for studies that have considered the addition of other mineral types, with an emphasis on absorbance of UV light. As shown, there has been quite a lot of research done on cerium oxide (CeO₂), which has been found to be an effective absorber of UV light.

Table 18. Studies Considering the Effects of Adding Other Inorganic Agents to Exterior Wood Coating Formulations

Organic UV Absorbers

As an alternative to the mineral particles just considered, e.g. TiO₂, ZnO, and CeO₂, the other main class of candidate UV absorbers that can be used especially in exterior paints can be described as organic UV absorbers. Within that category, there are a wide range of choices. Some of the most widely studied include benzotriazole, hydroxyphenyl-s-triozine, and hindered amine light stabilizers (HALS). But in addition, studies have also considered the usage of lignin, carbon particles, dihydroxy benzophenone, stains, and various other compounds as UV absorbers for addition to external coating formulations. Figure 18 shows the molecular structures of three of the most often studied organic compounds or classes of compounds used to protect the underlying wood from UV rays.

Fig. 18. Three classes of UV-absorbing components that can be used in coating formulations

Benzotriazole

As shown in Fig. 18, benzotriazole can be described based on a pair of resonance structures, one of which contains an aromatic ring. As is well known, compounds that have sufficient conjugation (double-single-double, etc., carbon-carbon bonding) will absorb light of various wavelengths (Yuan et al. 2018). The relatively short conjugation sequences present in at least one of these structures are apparently well suited for the absorption of UV light in a range that can be effective for external coating usage. Another attribute that follows from the chemical structure of benzotriazole is a positive charge, due to the presence of a primary amine in its protonated form. The positive ionic charge can, in principle, favor association of the compound within negatively charged groups, such as the carboxylate groups present in typical acrylic latex products that utilize alkali-swellable latex products (Padget 1994; Larson et al. 2022).

Table 19 lists some studies aimed at evaluating the performance of benzotriazole in the formulation of external wood coatings. As shown, both benzotriazole and its derivatives (usually hydroxyphenyl-benzotriazole) were generally found to be helpful in reducing effects of weathering.

Table 19. Highlights from Studies Considering Benzotriazole as a UV Absorbing Agent in External Coating Formulations

Hydroxyphenyl-s-triazine

The generalized structure of hydroxyphenyl-s-triazine, indicating points of optional substitution of this class of compounds, is was shown in Fig. 18. Note that, like the benzotriazole just considered, this is a structure with conjugated double and single carbon-to-carbon bonds. In addition, the tertiary amines on the central ring will allow for the development of a positive charge due to protonation.

Table 20 provides some highlights of studies to evaluate the effectiveness of hydroxyphenyl-s-triazine in exterior coatings. Most notably, though it was found to be effective against weathering, the studies often found that other agents were more effective.

Table 20. Highlights from Studies Considering Hydroxyphenyl-s-triazine as a UV Absorbing Agent in External Coating Formulations