Paul's advice is very good. I have used a one-part polyurethane (i think it was Interlux Britesides)with very good results. Its easy to apply with a brush or spray, very durable and color fast, and reasonably priced....you can find it at any boating store.

BTW, if you brush it, you can get a finish that looks almost as good as a professional spray job.....after you spread the paint with a bristle brush, use a foam brush to "tip off" the surface...lightly drag the foam brush along the wet surface....the poly flows very nicely....and remember, thin coats work best..they don't run...

good luck