I recently completed a total transom rebuild on mine, and used Marinepoxy. It is by far superior to any othe rtype of resins. Costs a little more, but good above and below water.
That would be EPOXY resin. Is much stronger and bonds to everything, costs more than polyester(much more) but very well worth it. Also, don't forgret, polyester resin is NOT waterproof, epoxy is.
