Marine varnish is the best choice for treating outdoor wood. It adapts to a wide range of temperatures better than other variations, and it protects wood from sun damage.
Marine, or spar, varnish has a higher oil-to-resin ratio than standard varnishes. This additional oil makes it a long-oil varnish, which is softer than other varnishes. The varnish may become sticky when heated, but this softness helps the varnish expand with the wood. Many marine varnishes also have ultraviolet inhibitors, which help protect both the wood and its finish from sun damage. This makes marine varnish the best choice for outdoor wood treatment or for any wood that may experience temperature variation.