When prepping cycle tanks the easiest way to remove the old factory paint is a 50 grit grinder disc. Then hit it with a DA and some 80 grit. 3 good coats of a Urethane catalyst filler primer works wonders. Prior to block wet sanding, dust a dark guide coat over the area. If your scarthes are still there the guide coat will show them. I have even wet sanded out a few small dings that required nothing other than the filler primer. If you think the scratches are too deep. Use some 220 on a DA, then re-coat with the 3 and wet sand.