Remove all the paint from and around the dent area using the 150-grit sandpaper. Sand off the paint until you can see bare metal. You may use a grinder for this purpose if necessary.
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Now prepare the body filler. Mix the hardener to the base filler using a rigid plastic spreader on a flat surface. Remember to read the instructions listed on the can. Also, once the body filler is ready, you have little working time because it will harden very quickly, so you will have to work fast from here on.
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When the filler is completely hardened, start sanding the filler using 150-grit sandpaper. Sand the filler using broad circular strokes. Switch to 220-grit sandpaper when the filler is nearly smooth and continue to sand until it is completely smooth.
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Using masking tape and paper sheets, you need to mask off the area immediately surrounding the area that area on which you applied the body filler.
Now spray the filled area with automotive primer. Spray a light coat of primer and let it dry. Then spray two more light coats, letting the first one dry before you spray the other one. The masking tape that you applied in the previous step will avoid the primer from being sprayed on undesired areas.
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When the primer coat is completely dry, remove the masking tape and paper. Wet the repair area using with water. You can use a spray bottle for the purpose. Also wet the 400-grit sandpaper and using it, sand the primer in a straight back and forth motion. Be careful not to sand off too much primer. Sand the primer until it is smooth and your repair work is complete, except painting the repaired area of course.
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