Coatings and Preservatives for Wood

Introduction – Wooden substrates are sealed or painted for both protection and beautification.  This is accomplished with a variety of products that either enhance the natural appearance of wood or change the color and appearance altogether.  

Some types of wood have a more natural resistance to decay and rot than others. Heartwood is the center of the wood and contains extractives of chemicals that provide the wood with its natural durability.  Sapwood on the other hand does not provide durability. Woods such as cedar and redwood contain higher quantities of heartwood than sapwood and are more naturally durable than wood like pine, which has a higher sapwood to heartwood ratio. The durability of cedar and redwood makes them popular choices for exterior uses such as decks or siding. When pine and less durable woods are used, sealants, stains, water repellents, and coatings are applied to improve the durability and weathering. 

Each of the materials has its own advantages and disadvantages and must be selected with the service environment and end use in mind. The demands of floors and furniture are different than those of wood in interior and exterior exposures. When painting, it may also be necessary to use stain blockers to prevent discoloration of the coating caused by bleeding from the wood. 

A discussion of the types of products used to seal and paint wood for architectural and specialty uses follows, together with the advantages and disadvantages of each.

Architectural Coatings

When working with wood that will be outside there is much to consider. Wood has the capability of remaining like new when it protected from UV light and moisture. However, exposure to rain and sunlight can lead to warping, splitting and discoloration if not properly sealed and coated.  Extractives give wood its natural color and are held together with cellulose cells by lignin, but sunlight breaks down the lignin in wood. Once broken down the lignin can be washed away by rain. Without the lignin serving as a “glue” to hold the extractives to the cell, the wood discolors.  Warped and split wood is the result of many cycles of shrinking when dried and expanding when soaked. Furthermore, mildew and other fungal growth is prevalent in humid environments because moisture intrusion and can cause wood rot. A number of products are used to slow the effects of sunlight and water on wood. Clear finishes, stains, paints, and water repellant are all used to protect exterior wood.

Oil-Based and Water-Based Paints. Paint is very effective at protecting exterior wood. Its film is thick enough to prevent water from reaching the wood substrate and the pigment can prevent the penetration of UV light. When choosing a paint, the choice is between oil-based and water-based (latex and acrylic).  Oil-based paint is selected when the surface will experience a lot of wear, like decks. It is also preferable when coating weathered wood since it penetrates deeper than latex and bonds well.

When working with freshly sanded wood, latex and acrylic work well and are a good choice for wood siding because of their ability to “breath,” allowing moisture vapor from inside the building to escape. If moisture is not allowed to escape, which can occur if many layers are applied, problems like peeling can develop.  While latex is a good choice for siding, is does not provide a good wearing surface for decks. 

Stain. Stain is commonly used to protect and beautify wood. Stains can also add color and protection from elements such as ultraviolet light and water.

Stain can also effectively protect exterior wood from degradation. Just as in paint, stain has a binder that resists moisture intrusion and can have pigments that resist UV light. Clear and semi-transparent stains allow for the wood grain to be visually enhanced and visible through the stain.  However, they contain less pigment and binder than paint, resulting in a thinner film that provides less protection.  Solid stains contain more pigment and provide more protection against UV light, but it is still a thinner film than paint so the long-term protection may be reduced.  However, because it is a thinner film, more coats can be applied over time without removing the underlying layer(s), making maintenance easier. Stains are popular for decks and wood fencing.

Clear Finishes. Clear finishes such as marine varnish can provide good protection against water penetration and sunlight since a UV absorber is added. While the UV absorber protects the wood from sunlight, the finish can still experience degradation like chalking, so more frequent maintenance may be required, limiting its use.

Water Repellant. Water repellants utilize materials such as oil or wax to repel water and improve the durability of the wood. Repelling water inhibits the ability of fungus to grow and prevents the wood from becoming discolored and experience decay or rot. Fungicides can be added to water repellants to create a water repellant preservative, enhancing the prevention of fungus growth. Water repellants also help to minimize the extent that wood shrinks and swells, reducing cracking and warping.  It is also possible to apply some types of water repellants to painted wood, to help prevent peeling and blistering.

Table 1 below provides a comparison of some of the above considerations as a general high-level overview, but the attributes of the specific brands of interest should be thoroughly reviewed before making a final selection as there will be differences within the same generic types.

Wood Finishes for Specialty Use

Finishes for specialty use, such as furniture, can be penetrating or film forming.  Penetrating coatings consist of pure oil and are not hard when cured.  Film forming coatings cure hard and are applied heavier than the penetrating coatings, and often serve as topcoats.

Film forming coatings for speciality use can be divided into five different types: shellac, lacquer, varnish, two-part, and water based. 

Shellac. Throughout much of the 19^th century, this unique, naturally forming resin had a corner on the wood coatings market for furniture. Today however, shellac has been almost entirely replaced by other finishes, primarily nitrocellulose lacquer. Shellac is an alcohol-soluble resin that is the product of the female Lac bug (Kerria Iacca, also known as Laccifer Iacca). The Lac bug secretes shellac onto branches across its native south Asia. Millions of these tiny insects can be found on a single branch. The resin is collected, heated, filtered, and formed into flakes. The flakes are naturally a dark orange color and have around 5 percent wax concentration. The flakes are then dissolved in denatured alcohol for use. A typical mixing ratio is 1 pint of alcohol for ¼ pound of shellac flakes.

Although shellac is not nearly as popular as it once was, it has some benefits that keep it relevant today. One of the benefits is that it bonds well with oil, wax, and resin. Like several other finishes, shellac is also self-sealing. Because it provides an excellent seal, it is commonly used as a sealer. The fine art of French Polishing (the application and rubbing of multiple thin coats of shellac onto the surface), is used more today for repairs than it is for new coatings. Additionally, since denatured alcohol is used as shellac’s solvent, it is less harmful to breathe and has less odor when compared to varnish and lacquer. It also stands up well against silicone.

Despite its positive qualities, shellac is now used primarily in niche areas such as French Polishing and as a sealer. This is due to some of the negatives associated with shellac. While its durability was excellent for its time, it no longer compares with the durability of more modern finishes such as lacquers and polyurethane varnishes. Shellac also has poor resistance to both heat and water. Finally, shellac only has a shelf life of 6 to 9 months, much shorter than lacquer for example, which has a shelf of 2 to 3 years.

Lacquer. Lacquer was developed in the early 20^th century and quickly supplanted shellac as the most widely used wood finish. Lacquer to this day remains one of the most widely used finishes for wood furniture. It is popular due to its ability to dry quickly and provide clarity, allowing the beauty of the wood to be displayed. It also has excellent rubbing properties, allowing for the easy removal of irregularities such as orange peel and brush marks. The most defining component of lacquer is its binder. The binder found in lacquer is mostly nitrocellulose. Cellulose is an organic compound that is typically extracted from cotton or wood pulp. Nitrocellulose is formed by treating cellulose with nitric acid. The resulting compound is highly flammable and can be used as a propellant. As the binder in lacquer, nitrocellulose allows the lacquer to have a quick drying time.

The resin is another critical component in lacquer. Due to the lack of flexibility and poor bonding of nitrocellulose, the addition of a resin is essential. Many different resins are used in lacquers to create a variety of products. Common resins include alkyd, maleic, acrylic, and urethane.  

The versatility of lacquer thinner with its different blends of active, latent, and diluting solvents is also a benefit to using this material. The evaporation rate can be varied by the thinner blend, allowing lacquer to be applied in almost any weather condition. Active solvents dissolve the lacquer, latent solvents in the presence of active solvents also dissolve the lacquer at a lower cost, and diluting solvents dilute the material, allowing it to be sprayed.

Drawbacks to using lacquer as a wood finish are similar to shellac.  Lacquer does not have as much resistance to heat and water as a finish like a polyurethane varnish. It also does not provide as much protection as other film finishes. Of the five types of film finishes, lacquer only offers more protection and durability than shellac. Lacquer is also prone to application defects such as blushing and fisheyes.  It also uses solvents that are toxic to breathe and have very strong nauseating fumes. It is also not considered to be environmentally friendly because of the high VOC concentration.

Varnish. Varnish is widely used on surfaces like floors. It protects the substate like any other coating, but forms a harder, more scratch resistant layer than shellac and alkyd. Varnishes typically create a clear coat finish with a slight amber effect. These qualities make a varnish particularly popular when coating hardwood floors and other surfaces that experience heavy traffic. Varnish consists of oil and resin, with metallic driers added to help accelerate oxidation and thus the cure time.  Different oils and resins can be used with the oil-to-resin ratio having a great effect on the firm properties. For example, more oil creates a softer film. The resin can also change the color. Popular resins include polyurethane, alkyd, and phenolic. Of these, alkyd is the least tough, but it still provides excellent resistance to heat, wear, solvent, acid, alkali, water, and water vapor, and is less expensive. Alkyd resins also tend to experience less yellowing over time. Polyurethane resin provides the toughest finish of these resins.

While the protection afforded by varnishes is excellent, there are also shortcomings. It is rare to find varnish used on large projects or by professionals where spray equipment is needed to work productively.  While varnish is easy to brush-apply, it is very difficult to spray. It also cures very slowly, even with the addition of metallic driers, so it can be prone to picking up airborne dust. Finally, even though varnish brushes easily, it can be difficult to achieve an attractive finish without a skilled hand. Problems such as brush marks and wrinkles in the topcoat, in addition to the dust are not uncommon.

Two-part finishes (Clear). Two-part finishes cure as the components react together after mixing and are used on floors.  The protection is as good as varnish but with a very quick curing time. They also use less solvent than many other finishes, making them more likely to be VOC-compliant. Some of the most common two-part finishes are epoxy and polyurethane. Epoxy resin is a unique finish because of its thickness. Because it is so thick, it is commonly applied by pouring rather than methods like brushing or spraying. Because it is poured, it should be used on horizontal surfaces such as tables. Two-part polyurethane provides a very durable finish. This is because the resin is 100 percent pure polyurethane. Catalyzed finishes are created when an acid catalyst is added to alkyd and amino acids. This combination cures to a hard film.

Despite the advantages, two-part finishes have their shortcomings. Making repairs and replacing these coatings can become challenging as they are very difficult to strip. Other modifications to two-part finishes such as adding decorative color is also difficult. Finally, the finishes use hazardous chemicals and fumes.

Water-based. A final finish is water-based. The ability to create water-based finishes has been around for decades. The same technology used to create latex-based paints is utilized to create water-based finishes. Water-based finishes have always been more expensive than other finishes and have historically been more difficult to apply. Despite this, there is a fast-growing market for water-based finishes today because modern additives have drastically reduced the difficulty in applying these finishes. In fact, it has become a popular alternative that is used by about half of all woodworkers.

Lack of solvents has added to its popularity because the amount of VOCs in coatings is restricted. The lack of solvents also helps to reduce odor and allow the finish to be non-flammable during application. They also have a high solids content, around twice that of solvent based finishes, improving film build in a single coat.

However, depending on the end use, the durability of water-based finishes may not be suitable. The resistance to water, heat, scratches, solvent, acid, and alkali is moderate and comparable to a nitrocellulose lacquer, but these properties are improving with the advent of new additives.  Scuff resistance is very high.

The appearance of water-based finishes must also be considered. Dark and dark stained woods tend to have a bland washed out look. On lighter colored woods, water-based finishes may not provide adequate color. Color can be added to the finishes, but it requires more steps. For example, dye can be added, but it needs to be carefully measured.  Alternatively, depending on the application, substituting a latex for the water-based finish may provide the color and good UV protection that is desired.

Other cautions with water-based finishes include the potential for wood grain to raise when in contact with water. This can be mitigated by dewhiskering prior to applying the finish, which is essentially raising the grain in advance and then carefully removing the raised portion, but dewhiskering adds a significant amount of time to the process. Slower dry times also contribute to runs and sags when they are applied to vertical surfaces. Finally, water-based finishes can be very temperamental in extreme weather. When applied in areas that are very dry, wet, cold, or hot, problems such as foaming can occur. Table 2 below provides a comparison of some of the above considerations as a general high-level overview, but the attributes of the specific brands of interest should be thoroughly reviewed before making a final selection as there will be differences within the same generic types.

Conclusion – There are numerous ways to finish wood and the options can at times be a bit overwhelming. All finishes have both benefits and shortcomings. If the most important characteristics of the finishes and service environment can be determined in advance, the selection becomes simpler. One of the most important attributes of a finish is its appearance. In instances where the wood grain is to be on full display, a transparent or semi-transparent stain or clear material would be considered, recognizing that all finishes, other than perhaps wax, affect the color of the wood to some degree.  Other times an opaque coat may be desired so a paint or solid stain may be the options.  Resistance to the environment – water, water-vapor, wear, heat, ultra -violet light, solvents and chemicals must all be considered when choosing a finish. Ease of application is another consideration when making a selection. Does it spray or brush easy? Does it cure slow making dust a concern? Safety is another consideration when choosing a finish, both for personal health and for the environment. If future maintenance is a concern, the ease of removal or repair needs to be considered. 

The topics in this wood series:

Topic 1 – Preparation of wood for painting

Topic 2 – Coatings and preservatives for wood

Topic 3– Inspection of cleaning and painting