A.Mark Knaebe responds: Treated lumber is not the source of your deck-painting difficulties. Clean, dry CCA-treated lumber is actually a better surface to paint than untreated wood of the same species. The real problem is that a paint’s performance suffers on a horizontal surface that is exposed to the weather. To make matters worse, deck boards are usually flat-grained, high-density wood that doesn’t hold paint as well as edge-grained, low-density wood.

For exterior decks, you’re better off using a water-repellent preservative or a penetrating-type semitransparent pigmented stain. Solid-color stains and film-forming paints aren’t recommended for horizontal surfaces because they may fail early. Hard enamel paints lack the flexibility to accommodate the movement of exposed wood. Flexible latex paints are not tough enough to stand up to foot traffic.

In contrast to paint, which flakes and peels, stains "erode," or wear away. Stains must be reapplied more often than paint, but it’s an easier job because there is no need for extensive scraping and sanding. Also, weathering stain is less of an eyesore than failing paint.

Sheltered porches can be painted with porch and deck enamel. First, treat the deck with a water-repellent preservative (check the label to make sure the product contains a preservative and is paintable). Second, prime the wood with enamel diluted with paint thinner. Last, apply two topcoats of straight enamel. Railings, whether exposed or sheltered, can be painted with latex paints.

Remember, wood to be painted should be dry but not exposed to more than a few weeks of sunlight. The sun’s ultraviolet rays damage wood fibers and weaken the wood’s ability to hold paint.

Mark Knaebe is a research chemist at the USDA Forest Products Laboratory in Madison, Wis.