How to Fill Trim Holes in Car Body

Materials

Clean the area around the trim holes with a degreaser. This will help the body filler to adhere properly.
Mask off the area around the trim holes with masking tape. This will protect the paint from the body filler.
Mix a small amount of body filler with hardener according to the manufacturer’s instructions. The mixture should be thick and creamy, but not too runny.
Apply a small amount of body filler to the trim holes with a putty knife. Make sure to fill the holes completely.
Let the body filler dry for the amount of time specified by the manufacturer. Once the body filler is dry, sand it smooth with 80-grit sandpaper. Be careful not to sand too deeply.
Apply a second coat of body filler to the trim holes. Let it dry and sand it smooth again with 80-grit sandpaper.
Finish sanding the trim holes with 120-grit sandpaper. This will give the surface a smooth finish.
Remove the masking tape and clean the area around the trim holes with a degreaser.

If the trim holes are large, you may need to use a backer behind the body filler. This will help to prevent the body filler from cracking.
Be sure to let the body filler dry completely before sanding it. If the body filler is not dry, it will be difficult to sand and may not adhere properly.
Use a light touch when sanding the body filler. Sanding too aggressively can damage the paint.