Shellac is actually completely waterproof in high concentration, and if you mix it really thick they used to use it as an adhesive/bedding compound for deck fittings.

So- you can buy epoxy in a can or mix it from flakes. In either case you'll probably dilute it with alcohol. When you read about it they will say, "one pound cut," or "three pound cut," or something similar. It's an "old fashioned" way of measuring the dilution based on the weight of flakes dissolved in one quart of alcohol. You want to start with a light dilution and add multiple coats. It's hard to guess what you should use, because it depends on the wood and the darkness of the stain. Just be sure you sand first so you don't highlight any glue squeeze-out.

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