Why does pressure-treated wood need to be coated with a preservative? If it's so dang tough, why bother?
Though the infused preservative prevents rot, it does not inhibit weathering...
The effects of the elements on pressure-treated wood are no different than with ordinary wood. So a preservative is a must, and should be applied as soon as possible after your project is completed.
Rapid drying causes warping, cracking and splintering...
Pressure-treated lumber is shipped to the lumberyard in stacks that are tightly bundled and damp... sometimes even wet. If you go and pick through a bin of pressure-treated lumber, you will see some pieces are straight, and others moderately to wildly warped. The warped pieces are invariably the pieces that were on the outside of the bundle... exposed to the sun and air and dried on one side. Once the bundle is broken they twist like Chubby Checker!
Once installed in your project and subjected to freely moving air and the sun, the same effect occurs. Shrinkage of deck boards can be excessive, in both length and width, and twisting can loosen railings and floor boards. Railings can become cracked and splintery, making them uncomfortable to use.
Applying a preservative slows drying, so less shrinkage occurs and the surface of the wood remains smoother longer. The preservative should be applied immediately upon completion of the project. Be sure that the preservative you purchase is recommended for use with pressure-treated wood. More about this in the next section...
Can pressure-treated wood be painted or stained? Definitely!
Stains first...
Many manufacturers carry full lines of both oil and latex products that can be used on pressure-treated wood. According to the folks at Cuprinol, you should wait at least one to two months before staining.
You may apply a clear preservative immediately, but it must be a product manufactured for use on fresh pressure-treated lumber. One such product is Wolman Oil-Base RainCoat Clear Water Repellant.
What about painting?
Don't even think about painting fresh pressure-treated wood! The moisture in it "stacks the deck" against good paint adhesion. Seal your project with a pressure-treated wood preservative immediately. Follow the preservative's instructions regarding future painting, making special note of the amount of time the preservative should weather before painting.
Applying a sealer can protect against CCA exposure...
According to the EPA, studies show that the application of a penetrating oil finish can reduce or eliminate exposure to CCA in older decks and to the chemicals used in newer decks. So it is recommended that all pressure-treated surfaces that have human contact be coated with an oil finish (or paint) as needed.
