I always use a good exterior varnish or a marine varnish that can withstand UV rays. I am currently refinishing 8 boards for a 51 and so far have applied 3 coats underneath and three coats on top. I cover the grooves and sides/ends also. Between each coat I use 220 grit sandpaper and on the final coat (which will be #5) in lightly sand with extra fine steel wool to eliminate any roughness such as a tiny bubble, etc. Comes out looking like glass.
