You want to seal the wood. You should cover the whole piece of wood with epoxy (before any metal is attached. Then you can put the metal on and varnish on top of a lightly sanded epoxy. Put whatever a few coats on top. When you want to strip the epoxy, use a little acetone so you don't mess-up the epoxy -- and recoat it with varnish.

The varnish should be UV resistant (exterior marine grade). If it doesn't work-out, you can always paint it. I would seal it first with epoxy in any event in a marine environment.