Does polyurethane make wood waterproof? - Quora
Think about a wood-topped bar. Sometimes you see them that have a really thick plastic like finish on top, making them totally waterproof and durable enough to withstand years of bar patrons; that finish could easily be polyurethane. there are lots of different polyurethane products on the market. What I'm saying above are will protect the wood longer. For boats, surfboards, and other projects that will be submerged, at least partially, look for a marine grade finish (usually an epoxy).