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Sealing & Painting Treated Wood By David Bowyer Preservatives make pressure-treated wood resistant to insect attack and rot, but the chromated-copper-arsenate (CCA) treatment does not prevent cupping, cracking, warping, or checking. This kind of damage is caused by water and sunlight, not biological activity (see "What Ages Wood?", next page). To shield the wood's surface from water and sunlight, and keep it looking good, you need to protect it with sealers and stains or paints. Sealers Sealers are penetrating coatings, generally clear or lightly tinted, that repel moisture. Their effective service life is only one to two years, with most manufacturers recommending an annual reapplication. Sealers make good undercoats for stains or good preprimers for paints. Many paint manufacturers are now recommending sealing before priming.