When I had my wood boat I had the benefit of having it docked at a marina which was also owned by a boat build/boat repair guy. He help me with ideas and tips on working on, repairing, and upgrading my boat. He got me into the West System epoxy and showed me how and where to use it. One thing was to always use marine grade plywood and he recommended West System epoxy. Marine grade plywood is more expensive but is top grade and it has no voids in between the plies.
He said that when using plywood, make all final cuts and to coat all sides with 50% thinned West System so it is absorbed into the wood sealing it and then coat again with full strength. After attaching the plywood, fill any voids and attachment points with West System to seal all areas.
After repairing and upgrading my boat using this tip I had the boat for over 20 years and had no problems with failure or rot. I always stained the plywood before applying the West System. I would top coat it with a high grade UV resistant varnish for interior usage and UV resistant marine grade paint for exterior usage.
I found two cons with West System. One, UV rays will break it down over time so it needs to be top coated with UV resistant paint or other UV resistant coatings. Two, It can't be used in any under water application.
Well, that was long winded...my point, I never had a issue with coating all side of the plywood in this application. Also, like I said earlier, I've used this technique on several other projects and had any issues.
There was one, I believe it was a SS member, who built a hab box on the back of his truck (I think it was a 7 ton) using plywood and West System.
But, to each their own.
Tom.