Body Filler, Plastic, and Composite Repair
Body Filler, Plastic, and Composite Repair tests substrate identification and the right chemistry for each material. Filler is not a substitute for straightening, and plastics are not all repairable the same way.
Body filler
- Apply only to clean, correctly prepared steel or an approved substrate; never over rust, paint, or e-coat without the specified prep.
- Epoxy primer improves adhesion; polyester filler is not waterproof and must be sealed.
- Follow maximum thickness; thick filler cracks and hides underlying movement.
- Shape before cure, then sand with a block and featheredge into the surrounding finish.
Plastics and composites
- Identify the plastic: PP/PE, ABS, PC, TPO, PUR, SMC, fiberglass, carbon fiber. ISO codes and OEM guides help.
- Flexible bumpers need flexible repair materials and adhesion promoters; some polyolefins require flame or plasma treatment.
- Structural composites require OEM-approved resin, reinforcement, and bonding; damaged carbon fiber may be non-repairable.
- Two-part adhesives need the correct mix ratio, bond-line thickness, and cure time.
Worked example
A TPO bumper cover has a tear. Reasoning: TPO is a polyolefin with low surface energy. Clean, identify, and treat the surface per the repair material instructions, use a flexible two-part repair adhesive with a reinforcement patch behind the tear, and refinish with a flexible additive. Rigid filler would crack.
Traps
- Using rigid filler on a flexible bumper.
- Skipping adhesion promoter on TPO.
- Applying filler over rust or a moving panel.
- Mixing two-part products by eye instead of ratio.