Thanks again, everyone. I've decided to use Frisket film which is .002" vinyl with a low-tack adhesive. You cut out your letters or numbers and stick the film onto the surface and spray your paint. It is supposed to peel right up without leaving any residue. Here's the stuff:

http://www.grafixarts.com/frisket_film.htm

I also think that those vinyl cutting machines will probably cut this stuff. There's a guy named Bill Fulmer who cuts vinyl graphics for model planes. I've asked him if his equipment will cut the frisket.

http://www.customcutgrafix.com/

David