Marine epoxy is your best bet to seal wood. I have seen fiberglass used many times and it does work, but it is messy. Just remember that your first layer that goes into the wood needs to be a SEALER and not a SEALANT. Sealants, like silicone, will form a protective barrier over the wood, whereas a sealer will actually penetrate the wood, offering you more protection. Don't forget to water proof the outside of the cabinet at well.
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