Unlike ordinary wood surfaces that are subject to infestation and rot, treated lumber is resistant to premature decay. Through a reverse-vacuuming process, the lumber is infused with chemicals that significantly prolong the life of the wood. Unfortunately, when inexperienced amateurs try to stain treated lumber, inadequate absorption often leads to ugly results. Moisture can affect stain absorption, although rain has little to do with this problem.
Moisture and Stain
Professionals add stain to wood to protect it from the elements and enhance its overall appearance. To adequately protect wood, stain must permeate its fibers. If wood is wet, stain will not absorb. Never attempt to stain moist wood, or flaking will eventually result.
The same chemicals used to protect treated lumber from decay also greatly inhibit stain absorption. Because they are reverse vacuumed deeply into the interior of the wood, these chemicals act as a moisture barrier between the finish and wood fibers. Even when the lumber appears dry on its exterior, the moisture barrier may still exist deep within. If you apply stain to treated lumber before it has had a chance to thoroughly dehydrate, flaking will occur.
You can stain treated lumber if you've given the wood enough time to adequately dehydrate. Professionals generally wait at least six weeks before testing the wood for absorption. If you want any chance at an attractive, lasting finish, do the same. After six weeks, apply a small amount of stain to an inconspicuous area. If the stain absorbs, continue with the application process. If it beads or runs, wait another two weeks before testing again.
Because rain adds moisture to wood, it can interfere with absorption. However, because rain is not reverse vacuumed into the interior of the wood, it does not have nearly the inhibiting effect as the initial chemical treatment. You shouldn't stain any type of wet wood. However, if you've allowed the treated lumber to adequately dehydrate prior to being rained upon, you can begin the staining process 24 hours after the rain has ceased. Simply test the lumber for absorption before applying to the rest of the wood.