For bigger holes that you cannot weld I'd recommend Pettit flexpoly. It's a 2-part mixing tube/caulk gun, expensive but does the trick. If you want to mix your own for the right "kick" then look at West Systems 655 gflex. Small holes where a less thick or runny compound is ok then check out Systems Three quick cure 5. I buy all these from Fisheries Supply, did a similar rebuild this spring on a 17' tracker.