I'm no expert, but I think you could get away with using finger nail polish. I've also used superglue. Right now, I'm redoing some of the club's masks with black enamel paint. You can buy a little bottle for painting models, and its held up pretty well so far. I guess try to keep your mask from getting put in with sweaty whites and clothes may help prevent rusting (as long as you aren't somewhere very humid). You can always use steel wool to get the rust off, and then repaint over it. But simply painting over it now may be your best bet.
Also, foil masks are required to be insulated, so this is a good practice if you fence foil and thats your competition mask.
