There are two things that effect exterior finishes considerably more than interior finishes: sun and water. Because of this most finishes you would use inside won’t survive outside for very long. So with that said, what does work outside?
Finishes, both interior and exterior, come in two basic categories, film finishes and oil finishes. Film finishes build up a protective film on the outer surface of your furniture. These are finishes like urethane, varnish, lacquers, etc. Oil finishes are absorbed into cells of the wood and protect from within.
If you choose a film finish for an outdoor project, I’d recommend a good quality spar varnish, preferably one with UV protection. Spar varnish is different from a “standard” varnish, in that it is designed for exterior use. It is formulated to repel water and the detrimental effects of the sun. It is usually a gloss finish, but with some searching, you could probably find a semi-gloss or satin spar varnish as well. It does need to be maintained, with a new coat every year or two. This may require sanding any rough spots that have developed and a light sanding all over to give the new layers a tooth to hold on to.
Oil finishes work well outside as well, but you don’t get the same feel that you get from a film finish. Oil finishes let you feel the texture of the wood and give you more of a flat finish. Again, make sure you choose a finish that is formulated for exterior use. These are readily available in deck stains and the like. One advantage these have is that they come in several colors and opacity. Another advantage these have is that when it comes time to maintain the finish, you can simply clean off the piece and reapply a new coat.
So what’s the right finish for you? Well like most things in woodworking, it depends. It depends on the look and feel you want; as well as how much maintenance you are willing do in the long run.
Oh yeah, one more thing. The finish you choose is irrelevant if you start with a poor choice of wood, or worse, an old piece that is already starting to rot. That however is a topic for another blog.