One other thing. For concave surfaces, like a cambered wing's bottom surface, you can fix sand-paper with double-face tape to a large cylindrical bottle, or a short section of PVC pipe. Something about three or four inches in diameter is a good size, with something about an inch in diameter for finishing spots. I use a couple of emptied thousand-count ibruprophin bottles (I go through the stuff like candy sometimes) for the large size, and an emptied bottle of CA 'kicker' for the smaller size.