I use any resin I can get a hold of, usually Eastman or West System and combine it with West System 207 Hardener. 207 creates a more flexible epoxy that does not get brittle. Remember that it is important to use the proper hardener-to-resin ratio. The ratio is usually indicated on the label for the hardener. I have banged the shyte out of my kayaks and have never had a crack as you have described. Any banging I have done usually results in scratches or gouges but nothing like a bruise or spider web crack. Hope this helps.

Robert N Pruden