A. Bill Feist, an expert on wood finishes, responds: There are many advantages to using a paintable water-repellent preservative (WRP) on unpainted clapboard siding before priming and painting. The treatment reduces raised grain, checking, warping, and splitting, and also improves paint adhesion. A WRP inhibits mildew growth on both painted and unpainted wood, and will retard decay in above-ground applications. A paintable preservative will help improve paint performance on the more difficult-to-paint woods, like flat-grain southern yellow pine and other flat-grain wood species.

Use WRPs only on dry, bare wood. The treatment must be done when the temperature is above 50°F. You can apply the treatment by brush or by dipping. When brushing, allow two days of warm, favorable drying weather before painting. When dipping, a full week of favorable drying weather may be necessary before painting. If you don’t allow enough time for most of the solvent to dry from the wood and for the wax in the preservative to be absorbed, the paint may not cure or bond properly.

If you want to apply a WRP to previously painted wood, remove all loose paint, then brush the preservative into joints and unpainted areas only. Remove excess WRP from the painted surfaces with a rag. Again, allow two days of favorable warm drying weather before repainting.

Commercially available WRPs include Clear Wood Preservative (in the Cuprinol Group) from Sherwin-Williams and DAP’s Woodlife Classic II Wood Preservative. When shopping for a WRP, the key word to look for is "paintable." There are many WRPs that are meant to be used as wood deck treatments and natural finishes, but these are generally not paintable.