Hey Kona, my NEW 2000 Grand Prix got hit from behind shortly after I got it. Years later all the clear coat started coming off the back end of the car where the repairs were made. Removing old clear coat without going thru the base coat was impossible. I tried. Using 400 grit got the bad clear up where I could see it flaking, and made the stuff I couldn't see separating easily come up also, but 400 rips right through the base coat. I ended up sanding out more than twice as much as what appeared to be shot till I got to the clear that wouldn't fly right off. Then went 800 and 1,000 to get ready for paint. Anyway, I took the paint code located in the trunk and bought a quart from the local paint supply. Then cleared with some stuff I had laying around here. It doesn't look perfect, but it looks ok to me. I'm the original owner and don't plan on selling it anytime soon.
Bottom line, there was no sanding the base coat without cutting through it. Impossible. The base coat is wicked thin and comes right off if your using a grit tough enough to remove the bad clear coat.
EDIT: Added a pic, basically did from behind the doors back.