When finishing bodywork and painting your car, the last thing you want to see after finishing the job is the body filler showing through the finished paint. This is called body filler print through or bleed through. This can happen when the filler is built up too high and is higher than the car's surface, the filler was not sanded well and the texture is showing through or when the wrong type of primer is used and the filler soaks it up, showing through the primer and paint. Stop body filler print through before you paint so it doesn't occur.
Use a long sanding block with 120 grit sandpaper to sand areas with dried body filler. Keep the block flat against the surface and lead with the long edge. Keep the block horizontal and sand in X shaped patterns to make sure all of the filler is smoothed. Areas that are left higher than the surface of the car will show through. Body filler should only fill in low areas on the body, not stand higher than the original surface of the car.
Change the 120-grit sandpaper to 320-grit paper. This smooths the scratches left by the 120-grit sandpaper. The scratches can show print through also, which looks like scratches or paintbrush strokes underneath the surface of the paint.
Spray the area with two coats of 2k primer (also called polyurethane primer) and let it dry for 6 hours. This primer is a filler primer, which means it fills in the small scratches and nicks in the filler and can be sanded. Other types of primer are too thin and are very easy to sand through.
Sand the primer with the long sanding block and 320-grit sandpaper to smooth the surface of the primer. You should not see the filler through the primer, even after sanding. The area is now ready for paint.
Wear a painter's mask before spraying primer.