You've gotten exactly the right idea, but you don't want to use nail polish 'uncut' on the primers, if you were to get the uncut polish on the actual surface area the firing pin would strike, it has a potential to 'soften' or somewhat absorb the blow of the firing pin due to the actual thickness of the coating of nail polish. You want the primer to take as much of an impact as possible from your firing pin, etc.
If you were to cut the nail polish, 50/50 polish/acetone, (or nail polish remover) it'd go a long way, because only a wee dab would suffice. You'd also get the benefit of the mixture being almost as thin as water. Insofar as appearance goes, I suggest either a clear coat, or at the most a transparent color base/tint. If you look at ball ammo, on the primer end you're going to see the colored lacquer sealant the ammo factory uses on Military Spec Ammo. If you go this route, you're going to want a very tiny artist's paint brush, with perhaps 3 or 4 fibers in it, just enough to carry a droplet about 1/4th to 1/3rd the size of a head on a straight pin to start, and then you'll develop a feel for how much sealant to use.
Ok, next question, say I wanted to seal my primers to make them more weather resistant. Does anyone sell primer sealant or can I use nail polish or something similar? Thanks for your help.