I'm not sure what you mean -- are you saying that you're painting part A and part B is getting a fine dusting of whatever you're painting onto A?
If so, the first answer is that you should mask the whole damned airplane before you spray, no matter how small a spraying job you're fixing to do.
I'm pretty sure that the second answer is the same as the bug answer, only much milder: if it's just a dusting, then take some 800 grit and sand it off. Maybe 1200 grit. You were going to color-sand and buff anyway, yes?