Although untreated, aged, wooden siding conjures up the sleepy seaside resort towns of the Atlantic coast, the chance of your project taking on the silvery aura of a beach-front summer house on Cape Cod is slim. Researchers at the Forest Products Laboratory (FPL) offer the following advice from the FPL’s Centennial Wood Handbook.
Traditional exterior finishes either penetrate wood cell walls or form films on the surface. Penetrating finishes give a more “natural” look to the wood than film-forming finishes—that is, they allow some of the character of the wood to show through the finish. In general, the more natural a finish, the less durable it is.
Although leaving wood to weather to a natural finish may seem like an inexpensive, low-maintenance alternative to finishing, this approach leads to problems. Wood surfaces erode, some wood species decay, lumber is more prone to split and check, and in most climates in North America, exterior wood develops blotchy mildew growth.
To avoid decay, wood must be all heartwood from a decay-resistant species such as redwood or western redcedar and be vertical grain to decrease the potential for splitting, raised grain, and cupping. Heartwood is the wood at the center of the tree, extending from the pith to the exterior sapwood. The heartwood generally contains gums, resins, and other materials that usually make it more decay resistant than sapwood.
Only limited areas have a climate conductive to achieving a driftwood-gray appearance as wood weathers naturally; the climate along the coast of New England seems conducive to developing the silvery-grey weathered patina that some people desire. Even when the climatic conditions favor the development of a silvery-grey patina, it takes several years to achieve this appearance. Protected areas under eaves will not weather as fast as areas that are not protected, which leads to a different appearance at the top and bottom of a wall.
Do not leave composite wood products, such as plywood, unprotected! The surface veneer of plywood can be completely destroyed within 10 years if not protected from weathering.
Consider instead a penetrating finish such as transparent or clear water-repellant preservative (WRP), a lightly colored WRP, or an an oil-based stain. Natural oils, such as linseed oil and tung oil, can penetrate the microscopic cell walls of the wood and modify the properties of these cells. Cells that are modified with finish typically absorb less water and swell less than unmodified cell walls.
Although the effect will look slightly different than untreated wood, most oils will not leave a film on the surface of the material, unless they are high-solid content semitransparent stains (which may leave a thin film).