But that's just how I've chosen to skin this particular cat...
Like Mel said, it's all about what works for YOU.
...using epoxy (a two part resin) and cloth, rather than fiberglass and resin. I don't really see or understand the difference, other than some chemistry changes...
If I may...
Styrene based resins will eat styrene based foams (like what were using here). That includes the traditional 'stinky' fiberglassing resins and bondo. Epoxy is a completely different formulation - it's like trying to compare oil and water: sure, they're both liquid most of the time, but that's about it.
Epoxies will not eat styrene based foam and, being epoxy, will stick to it like white on rice. They are FAR more pleasant to use. Sometimes, one can develop a sensitivity to the hardener (Part B), typically manifesting as a rash, after prolonged or repeated exposure. That said, I've been using it for years with no more than rubber gloves and haven't had any issues....but in all fairness it has to be mentioned.
Epoxies are indeed brittle on their own, but that's why we reinforce them with fibers or fillers like glass cloth, microfibers, or microspheres (or sawdust, or carpenter's chalk, or graphite, or flour, or...). Epoxy is also tolerant of amateur joinery and gaps of as much as 1/4" can actually be stronger than a tight joint depending on the filler used.
Lastly, it's actually more economical for the amateur builder, despite epoxies' higher initial prices. The same bottles of resin and hardener can be used for gluing, filling, glassing, and even painting with the addition of additives with no worry about compatibility.
...and I want the extra sturdiness of having a thin ply on both sides of the foam.
If you are talking about 1/8 ply, a similar thickness of epoxy will be stronger in most aspects and will be waterproof. With ply, you are depending on your coating (paint) to keep water from delaminating the plies. Any delamination in an epoxy layup is either excessive heat or surface contamination. It's only with styrene based layups that delam is common (like blisters on boat hulls - an epoxy barrier coat is applied specifically to reduce blistering)
Lastly, epoxy can be applied on top of styrene based products, but not the other way around. If you try and seal your foam with epoxy to prevent bondo from eating it, the bondo won't kick off before the styrene evaporates out of it.
