It takes 5 or 6 coats of Tung Oil before it has any moisture resistance. Each coat take a long time to dry. Its protection is via penetration. There is a reason they use varnish on boats. I would use an epoxy finish, seals at the surface and last nearly forever. I have had a sink that I epoxied 30 years ago that still look about the same as the day I did it. Gave it away last year, still going strong in a new darkroom. Just my 2 cents