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
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.