Clear exterior finishes: finding the balance between aesthetics and durability

JCT CoatingsTech, Sept, 2004 by Tom Daniel, Marc S. Hirsch, Ken McClelland, Alan S. Ross, R. Sam Williams

Water-repellent sealers and clear wood-preserving finishes give some protection to the wood surface from weathering by minimizing the effects of water. The wax in the product ultimately breaks down on the exposed lateral surfaces, and the water beading properties decrease. The life expectancy of the water repellency of these products typically ranges from six months to one year on exposed surfaces. However, the water repellent absorbed in the end-grain will be effective for many years. Also, after the wood has gradually weathered, additional treatments may last longer because the weathered boards absorb more finish. The UV radiation can still degrade the surface causing the release of wood fibers and thus the erosion of the surface. Including a UV absorber or some other type of UV stabilizer in the formulation can slow this degradation for a short time.

ULTRAVIOLET-RESISTANT CLEAR FINISHES

The only way to slow the UV degradation of the surface is to incorporate a pigment or a UV stabilizer into the formulation. Thus, the UV-resistant clear water-repellent preservative represents the broadest category of clear natural finishes on the market. Pigments and organic chemical additives are used as light absorbers (or screeners) to protect the products' binders and the wood from degradation by the sun's UV radiation. The pigments that are used most frequently are either a transparent form of iron oxide (transoxide) or ultrafine titanium dioxide that reflects UV radiation. The organic chemical additives are of two types--UV absorbers (UVA) and hindered amine light stabilizers (HALs).

Ultraviolet absorbers are designed to absorb selective portions of sunlight, especially UV radiation, and dissipate that energy as heat. During this process, some absorbers are destroyed, limiting the effective life of the product. The HALs act as scavengers for the components of the finish (binder and pigments) during degradation and then block further degradation of the clear finish and the wood. Generally, UVAs, HALs, and pigments are used together and offer the best protection to the wood surface.

Many people feel UV-resistant clear finishes are the solution to the delicate balancing act of the aesthetic desires of the homeowner and the protective requirements for the wood. The UV-resistant clears are not as clear as water; they impart a slight color or tone to the finish because they contain translucent transoxide pigments or titanium dioxide. Thus, with UV-resistant clears, various shades of color are available. Transoxide pigments are translucent and thus highlight the natural grain, texture, and beauty of the wood. Transoxide pigments are also versatile; they are used not only in traditional solvent-and oil-based product formulas but also in the latest formulas that meet tighter air quality (VOC) regulations, such as oil emulsion, high solid oils, and water-based formulations. The penetrating and film-forming characteristics can vary considerably among these types of products. The life expectancy of a UV-resistant clear finish is typically twice that of other clear finishes and can range from one to two years on horizontal surfaces (for example, decks) and two to four years on vertical surfaces.