How do you intend to hold the balsa in place from underneath while the epoxy cures? I experienced sagging. Some sort of bracing is necessary. As chucklesR said, you absolutely do need the inside layer of fiberglass. You can save the inside skin, grind away any rotten balsa stuck to it and glue it back in place. Then you'd only need to glass the seams. But holding the layers in place will be your biggest challenge. As steated in the earlier threads, the one lesson, main lesson, I painfully learned was: do it from the top. I know, you know. Once the epoxy was applied, I didn't want to stop and grind it all away after I figured out how to brace, so I ended up putting screws from the top into my core to hold it in place after the sagging couldn't be stopped due to insufficient bracing from below. That left me with holes to fill from above anyway. Think it through. Folks have been successful doing this from below, but not me.