When it comes to sanding off the primer, switching colors is a nice trick to help assess how much you still need to sand. When you think you are close to finish, spray a very light coat of black primer (doesn’t have to be a solid coat, just an even one) over the top then start sanding. If you’ve filled the low places, all the black should sand off. If you have low spots, the black will guide you to those places. You can get Dupli-color Sandable primer in black. If you can’t get black, grey will do.