It’s a two-sided coin: What enables pressure-treated wood to last outdoors is precisely what complicates the process of painting it. To produce pressure-treated wood, the milled lumber (typically pine or cedar) is saturated with chemical preservatives. These chemicals minimize the wood’s natural vulnerability to insects and rot, but they also leave the wood rather wet. To paint pressure-treated wood successfully, therefore, you must be prepared to exercise a bit of patience. To paint pressure-treated wood before it’s ready is to waste a day’s effort. For lasting results, follow the instructions below.
First, clean the pressure-treated wood you plan to paint. Use a stiff-bristled brush and soapy water. Once you’ve given due attention to the entire surface, rinse off the wood and allow it to dry thoroughly. Between the chemicals used to treat the lumber and the water used to clean it, the drying time may be as protracted as a few weeks—or even a few months. How do you know when it’s ready? Once the wood feels dry to the touch, sprinkle a bit of water on it. If the water soaks in, then the wood can be painted. If the water beads up, go back to playing the waiting game. Note that for a time-sensitive project, it may be wise to choose pressure-treated wood marked as having been kiln-dried after treatment (KDAT). The timeline for painting KDAT wood is considerably more condensed.
Once you’ve confirmed that the wood has dried out completely, you can begin painting. Start with primer formulated for exteriors, and make sure that the manufacturer lists the coating as suitable for use on pressure-treated wood. Having primed the wood—and having allowed sufficient time for the primer to dry (it should take no more than a day)—move on to applying your top coats. You should expect to do two. Avoid using oil-based paint here; on pressure-treated wood, latex performs much better. Use a paint sprayer if you have one, but if the job entails detail work, opt for a brush (or use both in combination).
It’s worth mentioning that in outdoor applications where the finish will be subject to the elements, paint lasts longer on vertical surfaces like fences than it does on horizontal ones like decks. If you don’t like the idea of repainting every two or three years, consider staining the pressure-treated wood instead. Yet another option is to allow the wood to weather and become gray, and then to coat it with a protective sealant. Of course, sealant must also be reapplied, but many consider the job to be less demanding than repainting, which often entails scraping away parts of the old finish.