I didn't cover my balsa with fiberglass, although its probably a good idea, since I figured it would be much stronger than the open cell foam that comes with the kit. I did epoxy varnish it, 3 coats, and of course it gets covered with several layers of fabric. One thing I did do when I went to the hobby shop to purchase balsa blocks was to choose the more dense, heavier, ones rather than the soft light weight ones; there is quite a difference in balsa.