I read an article, many years ago, written by a good-guide on the East coast - north of NY. He guided for Brant and Canada Geese (I think Canada Geese),
and found with the fall rains, his shotgun needed re-finishing by mid fall, every year.
Over the years of trying all of the various products, he finally settled on Spar Varnish, which was the ONLY finish he could find that would last the whole season,
so he re-finished his gun, ever winter and spring, allowing 3 to 5 days for each coat to harden before applying another.
That's when I switched to spar varnish for my Citori's fall hunts and work, it did. When the steel shot laws came in, I sold it & have not needed a spar varnish finish since.

It works for severe inclement weather. I suspect even the factory plastic/epoxy finishes are not as good.