by crumvoc » Tue Mar 06, 2012 1:00 am
I agree with the comments that it is not the epoxy. It is, however, how you USE the epoxy. I have built a couple of plywood boats of the "stitch and glue" variety. This method uses butt joints, temporarily wired, or otherwise mechanically joined, then permanently glued/sealed with generous applications of fiberglass cloth, tape and roving embedded in marine epoxy. Using that method, which is time consuming and fairly expensive, I have not had any plywood failures, including one boat that has lived it's entire life (12 Years) outside, not under cover (but stored upside down.) A great resource is Wooden Boat Magazine (a great magazine, by the way. WF Buckley once said it was the best written periodical in the English language), and Sam Devlin, a Seattle area designer and builder who has written a couple of very good books on the subject. Any other method of joining butt /edge joints is, in my opinion, bound to fail sooner, rather than later. Once moisture is introduced into the plywood, it is just a matter of time before you end up with a tasty plywood soup.
Regarding delamination on flat panels, this seems to generally be the result of a "glue starved" laminate. If one applies fiberglass cloth and epoxy directly to a raw surface, the wood will suck up some of the epoxy, resulting in a weak bond. The recommended method seems to be to apply epoxy to the raw surface, let it cure, wash off the blush, sand and then apply the cloth laminate construct.
Regarding the note from "Working on it" above, voids in plywood are also the kiss of death. Marine plywood, the good stuff, is highly QA'ed and engineered to eliminate voids. A void in plywood is just a vacuum waiting to be filled with water...
Living in the high-desert.