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...

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