More food for thought:-
The planked deck of my latest model was covered with finest grade GRP wing tissue, which, after rubbing down with fine wire wool, came up like a natural wood unvarnished deck which will never succumb to water ingress .
If a 'conventional' varnish finish is preferred, use a Matt variety - though after several coats it will come up much darker than a GRP tissue finish, and will not be as waterproof.
For what it's worth, I intend to use the GRP tissue method on all future decks, irrespective of whether unpainted or painted. In the event of the latter, I would overspray with a Matt vanish top coat.
