Plastic repair: how to repair bumpers and other plastic parts
Heating, gluing and welding — the techniques that save a plastic part instead of replacing it
Introduction
Bumpers grille and other bodywork parts are increasingly made of plastic and composite instead of steel. It makes cars lighter but it also sets new demands on the car painter because plastic behaves differently than metal. Fortunately most breaks cracks and dents in plastic can be repaired so the part will hold and look like new — often cheaper and faster than ordering and painting a whole new part.
Why repair instead of replace?
A repair is typically cheaper for the customer or insurance company goes faster because there is no need to order and paint a new part from scratch and is at the same time a more environmentally friendly choice because the old part is not discarded. For simple breaks cracks dents and scratches on a bumper Danish workshop prices typically show a level of a few thousand kroner — far below the price of a new lacquered part.
Three basic techniques
- 01Heating: minor deformations and dents in the plastic can be gently heated and shaped back
- 02Gluing (bonding): a two-component adhesive bonds fractured surfaces together — correctly executed, a glued joint holds as well as a new part
- 03Welding: the plastic melts and is formed together with filler material so strength and shape are restored before the surface is prepared for lacquering
| Technique | Usually used for | Important to remember |
|---|---|---|
| Heating | Smaller dents and deformations | Too much heat can damage the plastic's structure |
| Gluing | Fractures and cracks where the plastic type is suitable | Observe cure time and temperature from the datasheet |
| Welding | Larger fractures where strength is crucial | Requires knowledge of plastic type and correct welding rods |
Work process step by step
- 01Thoroughly clean and degrease the area of wax, oil and dirt
- 02Perform the actual repair: heating bonding or welding depending on the damage.
- 03Sand the transition flush and fill any holes with flexible filler
- 04Apply a flexible primer designed for plastic substrate
- 05Paint with a base coat and clear coat that contains a flexibilizer so the layer can follow the plastic's movements without crackling
Flexibility is the key word
A bumper flexes and springs a little even under small impacts and temperature swings. If you use ordinary, rigid filler or paint without a flex additive, the repair will sooner or later crack or come loose, even if it looks fine at first. That is why speciality plastic products — flexible filler, plastic primer and flex additive for the paint — are not a detail, but the very precondition for a durable result.
“Properly executed gluing or welding holds as well as a new part — the difference is not seen either in strength or in the paint on top.”
Documentation for insurance claims
When a plastic or composite repair is to be invoiced to insurance, it's important to include all elements in the damage assessment—both the repair work itself, the specialized materials used, and the extra time flexible products and curing times require. This topic is expanded on in the article about damage assessment and cooperation with insurance companies.