You need to get ALL the metal to look the same with NO black or darker spots left at all. It needs to be a nice shiney almost looking metal. If I were you, I would do each panel, fender, bed, etc at a time which consists of taking each one off, sand it down to bare metal, clean it good and primer it and put it aside and move on to the next part. That is so you don't get surface rust the next day because if you sand it to bare metal, most likely you cant sand it all down and primer it in one day and surface rust comes back overnight.

Good luck!