Years ago I made a rack for transporting fishing rods on the roof of my vehicle. It's made out of red oak and finished with (probably) several coats of polyurethane. I don't remember exactly what finish it was, but I will say that it did not hold up well against the elements (rain, salt air, etc.). It developed ugly black blotches. I'm eventually going to strip it down and refinish, but I'm looking for recommendations on a good finish for this application. Spar varnish maybe?