When I painted the numbers on 72G, I used a paint mask I bought from Moody Aerographics, (sorry Bengie). I used compressed nitrogen to spray with, no oil, water, or air compressor to deal with. I think the mask, paint, hardener, and all was less than $300.00. And yes, you have to peel the mask off before the paint gets hard to allow the edges to flow down and not leave a ridge in the paint. All the other advice is right on the money.
J. W. Palmer 1970 C-150K N6272G Ashland, VA (KOFP)