Exterior Finishes For Wood

The unique characteristics of wood make it suitable for a variety of exterior building applications. Without protection from sun and moisture, however, wood weathers and eventually deteriorates. Finishes are used to enhance appearance and dimensional stability, and to prevent deterioration. Recent research conducted by the Forest Products Laboratory indicates that failure to properly treat exterior wood can reduce the average life of the wood by 20 percent.

Understanding the differences between finishes makes it easier to select the right product. In the past, finishes were made from alkyd or natural oil resins, such as linseed, tung, soya and paraffin. The resins were often blended with waxes to provide additional water repellency, then diluted with a mineral spirits solvent.

Technological advances and environmental regulations on emission levels of volatile organic compounds (VOCs) have spurred the development of new products. Water-based products, particularly those formulated with certain water-reducible synthetic oils and resins, have excellent penetration and perform as well as or better than oil-based finishes.

The performance of commercially available wood finishes is often listed on a product label or in literature supplied by the manufacturer. The American Society for Testing and Materials (ASTM) has standardized test methods to measure the water repellency and color retention of wood finishes. Individual manufacturers also use outdoor tests to measure weathering of their products in various climates, and they might provide the results on request.

TYPES OF FINISHES Finishes are generally classified into two categories. The first forms a film or coating on the wood surface. The second penetrates.

Film-formers provide a barrier between wood and the elements. Film-formers include many alkyds, latex-acrylics and varnish resins in solvent or water-based finishes. Products without pigments are considered to be a clear or transparent finish and offer little or no protection from ultraviolet radiation. Pigments are added to paints, solid-color stains and semi-transparent finishes to change the appearance of the wood and protect from UV rays.

Some of the newer water-based coatings are semi-transparent acrylic blends that have excellent flexibility. Unfortunately, due to their higher molecular weight, acrylics still form a film on the surface and are subject to the cracking that is characteristic of all film-forming finishes. A film finish cracks as wood expands and contracts during normal moisture cycling, and water gets underneath the finish and deteriorates the wood. Generally, film-forming finishes are not recommended for decks.

Penetrating wood finishes are oil-based and water-based products that saturate wood pores to prevent water penetration. They typically contain a drying oil or resin in a transparent or semi-transparent stain. Penetrating finishes provide long-term water repellency but, unlike film-formers, they don’t trap moisture in the wood and do not peel or blister.

Natural oils (linseed and tung, for example) are initially very effective in stopping the absorption of water into wood, but they tend to darken over time because they are a food for fungi. Wood treated with natural oils and resins generally need extensive cleaning before reapplying the finish.

Some of the newer water-based systems have synthetic oils and resins, and they provide excellent water repellency and color retention. One of the main advantages of synthetic resins over natural oils is they do not serve as a food for most biological growth, making future coats easier to apply.

APPLYING FINISHES Whichever wood finish you choose to protect your exterior wood, correct application is critical to performance. Follow the manufacturer’s instructions, particularly with the newer water-based formulations. All finishes should be applied to a clean surface, but penetrating finishes must be applied to surfaces that are porous and free from previous coatings.

Although chlorine bleach will effectively remove many stains, including mold and mildew, it can damage wood and is toxic to people and plant life. Newer, chlorine-free cleaners are environmentally safe and can actually increase product penetration up to 25 percent. Wood that is pre-treated with a cleaner or pressure washer will probably have some raised grain, but it will also allow better finish penetration.

Water-based finishes tend to dry faster than oil-based products. To avoid lap marks, particularly on hot sunny days, apply these only in the shade. The cooler surface will absorb better and allow for easier application of a second coat.

MAINTENANCE Routine maintenance is necessary, but the lifespan of a finish depends on a variety of factors. Construction details, exposure to the elements, product choice, surface preparation and application techniques are all essential to success. Some finishes may even require chemical stripping or sandblasting to restore wood to the proper condition before re-treatment. Finishes that weather unevenly and are recoated without removing the old finish will have an unsightly, patchy appearance.

Although the wood finish is only a small percentage of the cost of a deck or exterior wood project, it is one of the most critical elements in these types of construction. To most homeowners, aesthetic appeal is just as important as performance when selecting a wood finish. Understanding the properties and expected performance of various products makes the decision process much easier.



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