what you want to do is either go to a hardware store and get fiberglass resin made by bondo. You also have the option of getting epoxy resin made by Smooth On or other companies. Smooth On in the most recommended. Give the exterior 2 coats and the interior 1 coat of resin. To reinforce the interior after the resin you can use epoxy putty or make rondo with resin and bondo mixed together. Then you can use the fiberglass cloth or mat for the interior. Do a little research on the process. Its actually easy.