Marine Spar Varnish - about 10 coats brushed on with light sanding between coats will give you a long lasting finish. My finish is on for ten years and it looks new.
Here is the key - you MUST do both sides and the edges of all of the boards. In other words, encase them or seal them totally!
That will do the trick. Look at my bed and side boards on the link below! It's perfect and I did it in 1997. I was told to do it this way by a paint manufacturing company tech.
Joe
