Pressure-treated lumber: to finish or not to finish?

I'm building some outdoor projects out of pressure-treated lumber and am wondering what finish, if any, I should use?
—Peter Vrooman, Canton, N.Y.

Peter, pressure-treated lumber (often Southern yellow pine) is infused with a chemical solution that makes it less susceptible to insect infestation and rot. This solution initially gives the wood its distinct green or brown cast, which weathers to gray after a few years of exposure to the sun and elements.

Although you often see pressure-treated lumber left as-is, no rule says you can't finish it. But keep a couple of things in mind if you choose to go that route.

Upon emerging from the pressure-treating process, the wood contains a lot of moisture that may interfere with stains or finishes. To determine if the lumber is dry enough for finishing, sprinkle some water on the surface. If the wood readily absorbs the water, it's dry enough to proceed. But if the water just sits on the surface, let it dry for a few days (or weeks) and check it again.

As with any outdoor project, choose an outdoor finish with UV (ultraviolet) protection that also applies easily, because you'll need to refresh it periodically. Most film-forming finishes, such as polyurethane and paint, crack and peel as the wood moves during the seasons, making them poor choices. Penetrating-oil finishes, on the other hand, soak into the wood, creating a flexible barrier to water and preventing the wood from drying out and cracking. But more important, many penetrating oils combine pigments with the finish, adding UV protection and allowing you to transform the green or brown hue common in pressure-treated lumber into something more attractive.

To apply a penetrating-oil finish, clean the wood first with a deck-cleaning solution and allow the wood to dry. First, test the finish on an inconspicuous area to see how the color looks on the treated lumber. Then roll, brush, or spray the finish. If rolling or brushing, maintain a wet edge to avoid lap marks in the color. If spraying, have a helper follow behind to back-brush the finish, ensuring complete coverage. Usually, two coats are sufficient. Reapply the finish every two or three years to maintain protection.

Deciding whether to finish pressure-treated lumber really comes down to the project and your personal preference, Peter. For structures such as fences or railings, skip the finish and let nature take its course. But to preserve decks or outdoor furniture, applying a pigmented penetrating-oil finish proves worth the time invested.