The correct grit sandpaper to scuff paint before repainting depends on the condition of the existing paint and the type of paint to be applied. Here are some general guidelines based on the search results:

• For scuffing old paint to repaint, use 150-grit sandpaper if the existing paint is smooth and in good condition.
• For rough sanding that needs to be done prior to bodywork, use 40 to 80 grit sandpaper.
• For flattening out scratches, feathering edges of body filler, spot putty sanding, and light rust removal, use 120 to 180 grit sandpaper.
• For pre-primer sanding, rough primer sanding, fine sanding spot putty, and final body filler sanding, use 320 to 400 grit sandpaper.
• For sanding surface imperfections in the primer prior to paint application, use 600 to 800 grit sandpaper.
• For removing base coat imperfections, use 1000 to 1200 grit sandpaper with the wet sanding technique.
• For smoothing out surface imperfections and scratches before and after applying the clear coat, use 1500 to 2000 grit sandpaper.
• Clearcoat may be sanded with 1500-grit wet sandpaper to remove small runs, dirt, and other minor imperfections.

Its important to note that wet sanding can be used with finer grit sandpapers to achieve a smoother finish and prevent clogging of the abrasive paper. Additionally, using a sanding block can help ensure even sanding and prevent uneven surfaces.