Yeah, sand the paint out PAST where your primer is going to end. So that your primer has adhesion all the way out to the end. You can mask it so you don't get overspray all over the place. But there is no need to mask it like that, just sand out further than you are going to prime. Something out on the outer edges like 180 or 240 or even a red scuff pad is fine. If you are going to do a nice paint job you will need to remove all that paint anyway and you can correct it all then.