Wire wheel down to bare metal till it's nice and shiny and rust is no longer visible, treat with ospho or similar to get in the pits, coat with a good epoxy primer or even rust bullet, prime, respray. I would guess you run a trunk mat? If so, the one thing they're good for is breeding rust, and does it fast at that. They hold/trap moisture. Even if you don't, B-body trunks are notorious for water leaks back there, thus your trunk a prime example. Anyways, don't try just some snakeoil rust fighter/encapsulator over the top of how it sits now...it will be back if you do. You need to neutralize it and the best way is to just get rid of it. Good luck