this probably seems like a dumb question, but here goes nothing..

i have an audi (1993, 90s) that i bought about a month ago, and the paint on the hood, roof, and a few other places is going through some unpleasant clearcoat failure, making what looks to me to be a potentially sexy car very pos-like.

so when prepping it, do i have to sand all the factory paint to look like the messed up paint does now (that is, the part that has undergone clearcoat failure)? or even as far as bare metal? or do i only need to sand of enough to make it a bit rough so the rustoleum can stick? or can i just get by fine without any sanding?

thanks