Polyester resin is used for laminating fiberglass reinforcement, molds and, yes, waterproofing wood. It comes in two types, finishing and laminating. Finishing resin is basically laminating resin with added wax that allows it to fully harden. Laminating resin will become hard but will remain slightly tacky to allow paint or more fiberglass to bond to it. Waxed or finishing resin can not be painted over without removing the top layer of hardened wax first. This is the most-used type of resin and can be found online or at your local marine supply store. LIke most other resins, it requires a catalyst in order to harden: methyl ethyl ketone peroxide.
Fill a small bucket with some polyester laminating resin. Add 4 to 6 parts acetone to the resin to thin it out and allow it to penetrate deep into the wood. Mix the acetone in using a stir stick until the resin is almost the consistency of water.
Add 12 drops of catalyst per every fluid ounce of resin and thoroughly mix it into the resin. You will have to mix a new batch of resin for each coat because it must harden in between coats. The resin will generally harden within 20 to 30 minutes.
Apply the first coat to the wood using a felt roller. Thoroughly cover the wood with the resin and then clean the roller out with acetone.
Mix another batch of resin with less acetone than the first batch. Add the catalyst to it and stir it up. Apply the resin and then clean the roller out. Apply at least 5 more coats with less acetone each time.
Mix a final batch of the resin adding 4 to 5 drops of the liquid wax solution. Add the catalyst and mix it up. Apply the final coat to the wood and clean out the roller. Let the resin cure for four to six hours and the wood will be waterproof.