Flaking bumper cover around a parking sensor
A 2020 SUV arrives with the rear bumper cover flaking around the left parking sensor. The paint has lifted in a rough circle about 200 mm across, the edge can be lifted with a blade, and there are two fine cracks radiating from the sensor hole. The customer had the bumper repaired after a car park knock about 18 months ago. A magnet does not stick, and an ultrasonic paint thickness gauge reads 380 µm on the repair area and 105–120 µm elsewhere on the cover. Through the wheel-arch liner the cover is marked 'PP+EPDM'. A quick wipe of the lifted edge with gun wash on a rag makes it soft and sticky. The shop's written repair specification allows a maximum filler thickness of 3 mm on flexible parts, and the painter wants to feather the edges, prime and blend rather than strip.
1. What is the substrate, and what do the magnet, the thickness readings and the solvent wipe actually tell you?
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The marking and the magnet result point to a polypropylene/EPDM bumper cover. The magnet only proves the cover is not steel; it says nothing about whether the surface is bare plastic, primer or paint, so it is not evidence of a sound substrate.
The thickness readings carry more weight. 380 µm over the repair against 105–120 µm elsewhere is roughly three times the factory film build, which is consistent with filler plus primer plus refinish. That is a thick, likely rigid repair on a part that flexes every time the tailgate closes or the car is washed.
The solvent wipe is the decisive finding. A fully cured 2K refinish should resist a few seconds of gun wash; a soft, sticky edge indicates a 1K product, an under-cured film, or both. Gun wash is also aggressive toward some plastics, so it is not a test to repeat on the visible surface.
Put together: a flexible PP cover, a thick previous repair, an adhesion failure at the edge and cracks at a stress point. The flaking is a symptom; the old repair is the defect.
2. Should the painter feather and blend, or strip? Justify the decision with the evidence and state the limits that apply to filler on this part.
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Strip the affected area and at least 50 mm beyond the lifted edge. Feathering would leave a weak, possibly 1K layer under the new paint, and the cracks beside the sensor hole would telegraph through the fresh clear within months.
Evidence for stripping: the edge lifts with a blade, the film softens with solvent, the repair is three times factory thickness, and the part flexes at the exact point where the cracks are.
Limits and checks:
- Filler must be a flexible bumper filler applied thin. The 3 mm limit exists because rigid polyester filler has no give; on a part that deflects, a thick or rigid skim cracks and the crack runs into the topcoat.
- Cracks must be traced to their ends, stop-drilled and, if the cover is torn, plastic-welded or the cover replaced. Filling over an open crack hides it for a season.
- Bare PP needs a dedicated plastic cleaner, not gun wash, and a plastic adhesion promoter. Ordinary primer will not grip polypropylene.
- Flexible additive goes into primer, sealer and clear at the maker's ratio, not 'a splash'.
- Adhesion test the surrounding factory finish before blending into it.
If the cracks prove structural, the honest answer is a new cover, not a thicker filler.
3. Outline the repair sequence from cover removal to refit, naming the products and the reason for each.
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- Remove the cover and any sensors; clean both sides with a plastic cleaner to remove road film and release agent. Record sensor positions.
- Strip the failed repair back to sound factory finish or bare PP, plus a 50 mm margin.
- Repair cracks - stop-drill, plastic-weld where needed, and dress flush.
- Abrade the repair area with 240–320 grit and feather the perimeter.
- Clean with plastic cleaner and allow it to flash fully. A wet cleaner traps solvent under the promoter.
- Apply plastic adhesion promoter to bare PP, then a flexible primer at the maker's film build. The promoter is what gives the primer a chemical grip on a low-energy surface.
- Guide coat and block to find highs and lows; the repair must be flat before sealer.
- Sealer in the recommended shade, then basecoat, blending within the cover rather than into the adjacent quarter, because a flexible-to-rigid blend line will show.
- Clear with flex additive, bake at the low temperature the bumper specification allows, and let the cover cool before refitting.
- Refit, reconnect sensors, and check parking-sensor operation and calibration if the maker requires it.