Polyester resin is a fibreglass resin that is used for a lot of general fibreglass jobs. Fibreglass will bond to many different surfaces including wood, styrofoam and even metal. Many times fibreglass is used to reinforce and protect wood. Fibreglass is a non corrosive waterproof material, that's fairly easy to work with. For reinforcement, two to three layers of fibreglass mat is applied and saturated with the resin. The mat will form and bond to the wood, creating a protective layer over the wood. Fibreglass resin is a thick liquid that requires a catalyst in order to harden. Fibreglass materials and supplies can be found at your local marine supply store.
Remove any rotten wood by sanding it off with 60 grit sandpaper. Sand the solid wood that is underneath the rotten wood smooth. Blow off any dust that is left on the wood.
Cut two to three layers of fibreglass mat to completely cover the wood. Use a razor knife to cut the mat to size.
Mix the polyester resin with the catalyst in a small bucket following the recommendations on the containers. Thoroughly stir the resin and catalyst together using a stir stick. After the resin is mixed you generally have 20 to 30 minutes before it hardens.
Wet the surface of the wood with the resin mixture using a felt roller. Apply the first layer of mat, saturating it with resin using the felt roller. Flatten the mat to the wood and remove any air bubbles using an air roller. Apply the rest of the layers following the same process for each. Let the fibreglass harden.
Put on a respirator and sand the fibreglass smooth using 200 grit sandpaper and a palm sander.
Always wear a respirator when sanding or cutting fibreglass.