Aluminum Fender, Blistering Body Filler, and a Plastic Bumper
A 2019 aluminum-bodied pickup arrives with three concerns: the hood has 40 rock chips and a 30 mm dent; the right front fender has a previous repair that is blistering in a 25 mm circle above the wheel arch with a rust-colored stain; the plastic front bumper cover has a 120 mm scuff and clearcoat peeling along the top edge. The vehicle has 96,000 km and lives within 2 km of salt water. The customer wants the fender 'scuffed and blended' and the hood 'touched up.' The technician has a magnetic thickness gauge, an ultrasonic gauge, a solvent wipe, and a heat gun. The shop's refinish system includes an epoxy primer, a urethane primer-surfacer, a sealer, and a plastic adhesion promoter. The fender's previous repair was not documented.
1. Assess each panel and decide whether to strip, feather, or replace. Justify each decision using the evidence in the scenario.
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Hood: 40 rock chips and a 30 mm dent do not justify replacement; feather or strip the chips to bare aluminum, repair the dent, and refinish. The aluminum substrate needs correct pretreatment, and a magnetic gauge will not read it. Fender: the 25 mm blister with a rust-colored stain is the critical item. On an aluminum fender, rust-colored staining is unlikely to be aluminum corrosion; it suggests ferrous contamination, a steel backing or attachment, or moisture trapped in the previous body filler. A blister means the old repair has lost adhesion, so scuffing and blending over it will not stop the failure. Strip the repaired area to bare substrate, remove all filler in and around the blister, and inspect for corrosion. Bumper: the 120 mm scuff and peeling clear require stripping the peeling area, feathering the edges, and refinishing with a plastic-specific system; replacement is not indicated unless the plastic is cracked or deformed. The coastal location raises corrosion risk, so any bare aluminum must be protected immediately.
2. What tests would you perform to identify the substrate and the condition of the previous repair, and how would you interpret the results?
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Use an ultrasonic coating-thickness gauge on the aluminum hood and fender because a magnetic gauge will not read aluminum or plastic; high readings over the fender repair indicate thick filler. A solvent wipe on cured OEM paint should not soften it; if it does, the substrate may be an uncured or incompatible finish. Gentle sanding can reveal the layers: OEM e-coat, primer, filler, and refinish. A magnet will stick to steel but not aluminum, and it will not identify plastic, so combine it with visual inspection and a flexible-part check. Inspect with a bright light at a low angle to show sanding scratches, pinholes, and the blister edge. Probe the blister edge with a pick: if the film lifts easily, adhesion failure is confirmed. The previous repair is undocumented, so assume nothing; trace the rust stain to its source before refinishing, because coating over it will not stop continued contamination or corrosion.
3. Outline the product sequence from substrate to topcoat for each area and explain the consequences of following the customer's 'scuff and blend' request.
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Hood: clean, strip chips, repair the dent, sand aluminum to the required profile, apply epoxy primer to bare aluminum within the recoat window, then urethane primer-surfacer, sand, seal, base, and clear; blend within the panel if needed. Fender: strip the failed repair to bare aluminum, remove filler, neutralize contaminants, apply epoxy primer, apply filler only over cured epoxy, then primer-surfacer, sealer, base, and clear; apply corrosion protection to the wheel-arch area. Bumper: clean with plastic cleaner, strip peeling clear, feather, apply adhesion promoter, flexible primer or sealer, base, and clear with the recommended flex additive. The customer's 'scuff and blend' would leave the blister and the rust source in place. Solvents in the new basecoat can soften the old repair, and the blister will continue to grow, causing a comeback within weeks. It also risks a corrosion claim on an aluminum panel and a warranty failure.