Bruce Hamilton replies: I suggest applying one of the exterior outdoor varnishes with UV inhibitors. Exterior varnishes will still need annual inspection for cracks or peeling. The surface should be cleaned with a mixture of one part ammonia to two parts water. Allow the surface to dry thoroughly or use a blow drier. Sand lightly for good adhesion. My shop sign faces southwest so I have to check it every year.
Bob Oswald replies: I prefer Linseed oil for a good outdoor protection. The first coat should soak in well and be put on heavy. I apply a second coat to brighten it up a few days later, wiped on, allowed to set for one half hour and wipe lean. All other hard surface finishes like varnish, lacquer and shellac will have microscopic cracks that increase over time from weather leading to failure. Simple oils like Danish don’t have enough ‘body.’ They dry and soak in quickly leaving little protection.
Donna Banfield replies: Use marine grade Spar Urethane, several coats. Another option, marine grade two-part epoxy, but this will be a bit more complicated to apply.