What's the best outdoor finish?

I'm preparing to build an outdoor furniture set, including chairs, tables, and other pieces. I'm torn between applying a film-forming finish, such as paint, opaque stain, or exterior varnish, or a penetrating-oil finish. What's the best route?
—Kevin Tundro, Humboldt, Tenn.

• Before we get into the specifics of each type of product, Kevin, keep in mind a few general principles that govern the performance of outdoor finishes:
• Two inevitable ravages of nature—water and the UV rays in sunshine—damage wood and the finish protecting it.
• Once damaged, wood fibers separate from the underlying wood, taking the finish with them.
• Nothing blocks UV rays better than pigments. Dyes impart some color to the finish but do little to block UV rays.
• Film finishes, especially betterquality ones that flex after curing, best protect wood from moisture.
• When film finishes fail, they require more maintenance than penetratingoil finishes.

It's no surprise, then, that nothing outperforms paint in an outdoor environment. Vertical surfaces may hold paint for 10 years or more, but paint on horizontal surfaces may peel within five years. Some products labeled "outdoor stain" are essentially thinned paint and perform nearly as well as heavy-bodied paint.

So-called "spar" or "marine" varnishes allow you to see the wood grain but come with distinct disadvantages. You need to apply five or more coats to build any level of UV protection. That protection might be short-lived, though, so be prepared to remove the finish every year or two, then reapply. Though boat owners might be willing to put up with the heavy maintenance, as well as the high prices for quality formulations, we don't recommend varnishes for most outdoor woodworking projects.

For a natural look, penetrating-oil finishes might be your best bet. High-quality formulations have finely ground pigments that block UV rays while allowing the grain to show. They shed water for a year or so before requiring reapplication. Prior to applying new finish, scrub the surface with a cleaner containing an oxidizing agent such as bleach. Then, rinse the surface with water and allow it to dry completely.

Ultimately, the choice comes down to how much you're willing to spend for the best-performing formulations, the importance you place on the wood looking as natural as possible, and how much maintenance you want to undertake.