L'l Joe- I used spar varnish for a number of years on my modern duck gun. I'd read an article written by a guide from New England who stated Spar Varnish was the only thing that was close to water proofing for his guide gun. As it was constantly raining most all hunting season, he needed as close to a waterproof finish as possble to keep the wood from soaking up water. The oils were useless according to him, as were the 'epoxy' finishes he tried.
He also noted he's re-finish his gun every year in the offseason to be ready for the next goose and duck season of guiding.
It takes a LONG time for each coat to harden is the only real detiment I found. The previous coat must be HARD before the next can be applied, or the finish will never harden, it seems.