A common question I get from do-it-yourselfers is how soon waterproofing can be applied to ACQ- or CCA-treated lumber after construction. The traditional belief is that the wood must weather 6-12 months before any finish is applied. Otherwise it will not stick.

Not so. The experts at the USDA Forest Products Laboratory (FPL) in Madison, Wisconsin, advise that wood treated with waterborne chemicals, such as  alkaline copper quaternary (ACQ) or chromated copper arsenate (CCA) can be painted right after installation before any weathering occurs (assuming the wood is dry). Wood treated with solvent or oilborne preservative chemicals, such as pentachlorophenol, is not considered paintable until all the solvents have evaporated.

The FPL suggests that for severe exposures and horizontal surfaces, such as decks and porches, a semi-transparent penetrating stain or water-repellant preservative provides the best finish. While the penetrating stain may be somewhat shorter lived than paint or solid-color stains, it is more easily renewed and doesn't peel or require the heavy scraping and sanding that paint does at refinishing time.

Light-colored penetrating stains will last longer than dark ones on exposed flat surfaces like decks, because they show the least contrast in grain color as traffic wear occurs. Refinishing is required every 1-2 years and is best done in the spring.  

Before staining, be sure to clean the wood of surface dirt, mildew and loose wood fibers. Deck cleaning solutions containing sodium percarbonate (oxygen-type bleach) are preferred for removing mildew, surface dirt and the gray residue caused by sunlight degradation. Oxalic acid-based products work best to remove tannin and iron stains from cedar and redwood lumber. Chlorine-based bleaches, while effective for mildew removal, do little to remove dirt or other surface deposits, and can result in an unnatural whitish color to the wood. Regardless of the cleaner used, rinse them thoroughly with clean water and let dry before applying the finish.

For more information, see the following FPL factsheets on the Web.

• What's in That Pressure-Treated Wood?
  http://www.fpl.fs.fed.us/documnts/techline/whats-in-that-pressure-treated-wood.pdf
• Cleaners and Restorers for Wood Decks
  http://www.fpl.fs.fed.us/documnts/pdf1998/ross98e.pdf 
• Finishes for Wood Decks
  http://www.fpl.fs.fed.us/documnts/finlines/willi95a.pdf