A.Bill Feist, a former wood-finishes researcher with the Forest Products Laboratory in Madison, Wis., and co-author of Finishes for Exterior Wood, responds: To protect the fir flooring over the winter, your best choice would be to use a paintable water-repellent preservative (WRP) like Behr's No. 2-85 Multi-Purpose Waterproofing Sealer (800/854-0133, extension 2; www.behr.com); Wolman's Woodlife Classic Clear Wood Preservative (800/556-7737, www.wolman.com); or Cuprinol's Clear Deck & Wood Seal (800/424-5837, www.cuprinol.com).

There are a variety of WRPs on the market. Many of them are formulated for use as natural finishes on decks and fences, but these are not paintable and could cause paint-adhesion problems like blistering or peeling later on. So it's very important that the treatment you choose be described as paintable by the manufacturer.

There are many advantages to using a paintable WRP on unpainted porch flooring before priming and painting. Besides inhibiting mildew growth and retarding decay in above-ground applications, the treatment reduces raised grain, checking, warping, and splitting. It also improves paint adhesion, which would be especially valuable on a difficult-to-paint wood like flat-grain fir.

It's always best to treat all sides (front, back, ends, and edges) of your flooring with the WRP before installation. But even when unfinished porch flooring has already been installed, it's helpful to back-treat with WRP as much as you can, assuming access under the porch is reasonable. Research has shown conclusively that solid wood products back-primed with a WRP retain paint better — and perform better overall — than those coated on one side only.