Pressure-treated wood is a favored building material because of its durability and its resistance to both moisture and insects. These wood products are made by forcing a chemical preservative into the wood under pressure. The chemical does its preservation job wonderfully, but it also requires that anyone wanting to prime the wood for painting take some special measures.
Test the moisture content of the lumber before priming. You should only prime pressure-treated lumber that has been allowed to age and dry out properly, and it will need to have had a drying-out period of at least 90 days since treatment for any primer to adhere well. Lumber can be tested with a moisture meter (a 12 to 15 percent reading or below is acceptable), or by making a test cut through the lumber with a circular saw. If the blade is wet after the cut, the wood is not dry enough for priming.
Clean the surface of the wood to be primed. If there is any mold or mildew present on the wood, pressure wash with a diluted solution of chlorine and water. Otherwise, just give it a good scrubbing with soap and water and a stiff bristle brush. Allow it to dry completely after the cleaning.
Apply a 100 percent acrylic primer to the lumber using normal painting methods of brush, roller or sprayer. Acrylic blends and other primers that are not 100 percent acrylic are not recommended for best results.
It is not advisable to use waterproof sealers on pressure-treated lumber. It will not absorb sufficiently into the wood, staying on the surface instead and attracting dirt, dust and providing an environment for algae, mold and mildew.