Filler primer would be a good idea if you have lots of scratches. I didn't do it, but I had a pretty straight body to begin with. The rustoleum will fill in sanding scratches (esp. with 18+ coats) to some extent, but for big scratches or gouges you can use a skim coat with bondo and fiberlgass resin, start the sanding with 180 which is not aggressive, and feather it into the surrounding area.

If you are concerned with perfect straightness, you might also investigate whether you'd be happy with the end result of the paint. It's not a show quality shine. So it would be worth doing some test panel first to see if it's worth all the prep it sounds like you want to do, then put a paint job over it that you might not like.