Check with a body shop. 80 grit is only used if you need to take down a lot of body filler, and I think you need to rough the entire car with 320 or 400 before painting. If the paint is peeling, you may find it easier in some places to take it off with a razor blade. It's just personal preference, really- if you like sanding, it works, but if you really hate it, there's an alternative.
Any bodymen here? Please reply- I haven't done this in a while.