Ideally you're only using rough paper once or twice ever in the life of the foil, specifically to get the manufacturing defects out when you first buy it and then again if you need to make a substantive repair. Otherwise, most sanding is relatively fine grit and is just to handle surface scratches and stuff. In that case, you're applying some sort of filler material like primer or Bondo and then only sanding until the filler is flush and blended with the adjacent surfaces. That's why Bondo is a better choice for most scratches because you can spot-apply it in a smaller area than spray paint so there's less universal sanding to get your desired finish.
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