You're describing a frustration that I go through on every boat. I built 1 plywood and four strippers and it always seems like you don't have enough epoxy on to do a good flat sanding without burning through here and there. I think that it's partly the time consuming process of laying on repetative coats and the will to keep the boat light. I'm running through that on the deck that I'm working on now.

Randon orbital sanders can easily sand through a rounded surface quickly and cut away the epoxy. I have learned that I sand very cautiously with the random orbital and sometimes switch to hand sanding and the pad sander. One other note: the pad sanders will generate more heat than a random orbital and will gum up an "almost cured epoxy" where the random orbital will sand it OK.

I'm now trying to be more patient and add more coats of epoxy.