You'd be better off just getting some fresh epoxy for building, as I don't think the finishing resin is made to hold stressed areas together. It's basically formulated to apply fiberglass cloth to wood or plastic, to flow evenly and to be easy to sand. These properties in my opinion make for a epoxy that's not necessarily strong enough to build with. You're going to need some regular epoxy sooner or later for a balsa model, so you might as well get some now.

Jesse