Depends on what type of material the bumper is. If its plastic, Dynatron (by Bondo) makes a "Flexible Bumper Repair Kit" which has been used successfully like here.
When I went to pick up the Dynatron kit, the body shop supply place didn't have it but said the SEM 39768 "Problem Plastic Repair Material" was similar. I used it to mold a lilliput into my dash bezel. It wasn't the most professional job because it was my first try and their were some problem inside corners and tight spaces. But, I can tell you this stuff sets up FAST and sands relatively easy...certainly easier than fiberglass.
This thread talks about the difference between the SEM and the Dynatron products.
Originally Posted by Arathranar