I have found the best to be fiberglass resin. Hardware stores carry these and they come with a small tube of liquid hardner. The viscosity is just like oil, so it can be applied in every nook and corner with a brush, it penetrates the balsa and strrengthens it also and makes it tremendously fuel proof. Look out for drips as it takes about half hour to become tack and drip free. But if you apply thin coats it will not run.