The easiest way to tell when you have sanded out the previous sanding scratches is to sand a different pattern when stepping up on paper. I start by straight line sanding then go to circular pattern. Once all the straight lines are gone from the circular path, you can move to the next grit. It takes some practice but after a while you can tell when to move to the next grit. The is my old 69 Camaro. BTW, I start at 600 grit when cutting a buffing. I also have applied 5-6 coats of high solids clear as well.
