Salvage Door with Unknown Repair History and a Used Plastic Bumper Cover
A 2018 aluminum-body pickup arrives with a salvage-yard right front door and a used bumper cover. The door's paint thickness readings range from 180 micrometers near the beltline to 620 micrometers along the lower body line, where the paint is blistered. A magnet does not stick at a bare-metal spot on the door edge, and the door shell has a galv stamp on the steel hinge reinforcement. The bumper cover has no visible ISO code, feels waxy, and has peeling paint at the lower lip. The shop plans to repair a 30 cm crease, refinish the door, and blend the adjacent cab corner. The customer wants OEM-level corrosion protection and a 3-year warranty. On the shelf are a magnetic and eddy-current thickness gauge, a magnet, plastic cleaner, adhesion promoter, epoxy primer, and a self-etch primer. The shop's DA sander was last used on a steel quarter panel.
1. What substrate and prior-repair conclusions can you support before sanding, and what tests would you run?
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The bare-metal spot that does not attract a magnet supports aluminum, not steel. The galv stamp is on the steel hinge reinforcement, so it cannot be used to identify the outer panel. Thickness readings above roughly 500 micrometers are too high for OEM paint and indicate body filler; the blistered area suggests a prior repair with poor adhesion or moisture trapped under filler. Before sanding, calibrate an eddy-current gauge for aluminum, take a grid of readings, and mark the repair zone. Use a magnet to separate steel from aluminum or plastic, and use a razor or controlled sanding test to confirm filler depth. On the bumper, look for an ISO code on the back; if absent, treat it as TPO or PP-EPDM and test adhesion on a small area. Do not assume the DA sander is safe: it was used on steel, so it can embed steel particles in aluminum.
2. Outline the preparation sequence for the aluminum door and the TPO bumper, including grits, cleaning, and primer choice.
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Aluminum door: remove trim and wash; strip the repair area to bare aluminum with 80-grit on a dedicated DA or hand block; do not use abrasives previously used on steel. Feather the paint edge with 180 then 320; keep the repair area free of filler if corrosion protection is required. Clean with wax and grease remover; apply the manufacturer's aluminum conversion coating, then an aluminum-approved epoxy primer within the tech-sheet window. Block with 320 to 400, seal, base, and clear. Avoid a steel self-etch primer unless its tech sheet explicitly lists aluminum. TPO bumper: wash, clean with plastic cleaner, scuff with 320 to 400, apply TPO adhesion promoter, then flexible primer or sealer; add flex additive to base or clear only if the product sheet requires it. Do not use a rigid primer on TPO.
3. What documentation and process controls would you set to support the 3-year corrosion warranty?
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Record substrate identification as aluminum, thickness readings before and after stripping, products and batch or lot numbers, conversion coating and primer application times, grits used, surface temperature, booth temperature, cure time, and photos of each stage. Use only OEM-approved or documented equivalent products and follow the OEM repair procedure. Keep the job card and tech sheets with the vehicle file. If a product's recoat window is exceeded, document the sanding and re-priming step. Warranties fail when the substrate or prior repair is undocumented, when steel-contaminated abrasives are used on aluminum, or when primer is applied outside its window.