There is no substitute!
Nothing sets off those little epoxy bumps or fiberglass nicks like laying on a really beautiful paint job and 3 layers of clear coat. It really makes those imperfections pop - not to mention the drag. Should have spent the time up front...
You can't finish the job without this stuff.
Start with the most coarse and work up to your finest. Don't be afraid to use fine sandpapers on paint. Very fine finishes can be accomplished with water and superfine sandpaper on paint and clear coat layers.