David Heineman, a fellow PC builder who lives down the road from me, has a question. He is having trouble getting on the forum, so I'm posting on his behalf. He caulked his interior seams generously with 5200, but also wants to glass the interior of the cockpit. He's wondering if it is ok to apply epoxy over the caulking. He wants to know if it will adhere properly. Any words of wisdom?
Epoxy will stick moderately well to 3M5200 or other polyurethane adhesive/sealants, but it doesn't like to to be disturbed. These sealants move around with temperature changes which will eventually break an epoxy bond or sheathing over it.
The short answer is yes, you can, but if the wood moves much, because it's not encapsulated, then it'll be fairly short lived. The bottom line is you can't water proof one side of a piece of wood with epoxy and fabric (or just epoxy alone) and expect the wood to remain stable enough to not shed the plastic coating in time. Wood will absorb moisture (which it does normally) and the expansion or contraction with differences in moisture content, will shear is plastic coating or fabric sheathing. This is why we encapsulate the wood (all the wood, every single piece, every single side) so the wood becomes stable and doesn't change it's size with changes in moisture content.