For smaller graphics using vinyl I seal them with clean fingernail polish. But if you are going to do a larger design that is going to add a lot of weight to the plane. I have seen larger planes use the printed shrink wrap so you would be ok there. However, those are applied over the primary covering and not used as the covering. Also, larger planes normally use gas engines that don't have the exhaust residue that glow engines do, so I don't know how the shrink is affected. If you have access to a vinyl cutter you can do quite a bit with monokote. These graphics on my Slow Poke are all Monokote and they were cut with a vinyl cutter, including the checkerboard on the tail feathers. By using monokote I'm assured that they graphics are fuel proof.

Ken