Natural Exterior Finishes for Wood in the Pacific Northwest

Natural exterior finishes are intended to preserve the natural appearance of exposed wood by protecting it from surface growth of microorganisms, from water staining and warping, and from ultraviolet degradation. Penetrating finishes which produce no surface coating and have little pigment are easy to apply and renew and can be formulated with water repellents. Forty-four experimental finishes, including water repellent preservatives, pigmented penetrating stains, and water solutions of inorganic salts, have been studied for from 2 to 8 years at a test site at Olympia, Washington. Water repellent preservatives containing fungicides, water repellents or waxes, and small amounts of resin to hold the materials in the wood and retard surface degradation, have been tested by varying the type of fungicide and resin. Results for about 15 formulations are tabulated. In tests of pigmented penetrating stains, an FPL natural finish containing 5 percent pentachlorophenol and 10 percent pigment with 60 percent linseed oil as a binder had excellent durability after 7 years of exposure, with one refinishing. This formulation was varied by increasing the penta fungicide content to 10 percent and decreasing the content of binders from 60 to 20 percent, with no effect on performance. Water soluble copper and chromium salts, as well as chromated copper arsenate provide excellent resistance to fungi and ultraviolet degradation. Ammoniacal solutions produce a nonuniform light green to brown color initially, but the appearance improves as the surface chemical residue weathers away. Tests of a 3 coat polyurethane varnish and a 2 coat polyurethane system did not provide durability suitable for exterior applications.

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