Q. I plan to install radially cut spruce clapboards on a new house and want to stain them with a white semitransparent or opaque stain. What type of finish appearance should I expect, and how often will the finish need to be renewed? Although stained siding looks better to me than painted siding, I know that some people say that stain is "just thinned paint."

A.Bill Feist, a wood finishes expert in Middleton, Wisc., responds: Semitransparent stains are most effective on roughsawn and weathered wood because more finish can be applied. The best exterior house stains are usually described as semitransparent, penetrating, and oil- or alkyd-based (solvent-borne). The better alkyd-based penetrating stains contain a fungicide (preservative or mildewcide), an ultraviolet light stabilizer, or a water repellent. Check the label for these important ingredients.

The alkyd-based solvent-borne stains actually penetrate the wood surface to a degree, and do not form a surface film like paint does. Thus, they don’t totally hide the wood grain and will leave a soft, flat appearance. They will not trap moisture that may encourage decay. Since they penetrate and don’t form a film like paint does, the stains cannot blister or peel even if moisture penetrates the wood. Alkyd-based stains normally only require a light cleaning with a stiff bristle brush and water before refinishing.

Latex-based (waterborne) stains are also available, but they do not penetrate the wood surface as do their oil- and alkyd-based counterparts. These are essentially "thinned paints." On spruce clapboards, a latex stain probably won’t perform as well as an oil- or alkyd-based stain, and could also be more difficult to refinish.

How long the stain will last depends on weather exposure and the roughness of the wood. When used on new smooth-planed siding that is fully exposed to the weather, semitransparent penetrating stains generally last only about two to three years. When refinished after weathering, a smooth-planed siding should accept two coats of stain, and the finish will usually last much longer than the first application.

Since a rough surface will usually accept two coats of stain, even on the first application, it is preferable to a smooth surface. Stain on roughsawn siding may last six to eight years, depending on the amount of exposure. However, such durability often requires applying the stain at a rate of 100 to 150 square feet per gallon, a much greater amount than usually required for paint, which is typically applied at a rate of 400 to 450 square feet per gallon.