Another little trick is this... Sometimes you come to a piece of balsa that has end-grain showing (usually a wingtip) that, no matter how good you sand it, it remains "fuzzy". Now, you could use some dope to harden it, But it's been years since I've used dope, so I don't have any around, so what I do is thin down some regular white glue (or carpenters glue) with water and brush it on the area. Let it dry overnight, and it cures the "fuzzies". Now, it will sand to a smooth finish.