It's important to use a SEALER not a SEALANT. a sealant, like silicone, will only cover the surface of the wood, while a sealer will actually penetrate the wood and protect it. Marine epoxy is the best for that, but it is pricey. I have heard about others who have used lacquers, polyurethane and shellac.

I have a large wooden enclosure build planned and I am going the marine epoxy route. I don't mind the added expense to ensure it lasts a long time.