I have been told that you can put a layer of resin (epoxy) on to the wood, then prime, sand and paint. I have never done this but I hear you can.

The other thing you can do is Fiberglass, sand, prime, sand and paint. You will find a number of thread on this site about how to fiberglass a plane, it is not to tough and it gives you a really nice finish to work with, but it will add weight.