Blisters on a Three-Month-Old Silver Bonnet Repair
Three months after a repair, a customer returns a silver metallic bonnet with blisters along the front edge and around a repaired stone-chip area. The shop's records show the bare metal was feathered, two coats of 1K etch primer were applied at 18 °C and 55% relative humidity, polyester filler was used on a 30 mm dent, 1K primer surfacer was wet-sanded with 800 grit, then 2K silver base and 2K clear were applied. The compressor has a refrigerated dryer, but the filter/regulator was last serviced 14 months ago and the line drain has not been opened. Air is delivered at about 4 bar at the gun. Blisters appear only over the filled area plus a few along the bonnet's leading edge, and scraping one shows a rust-coloured pit under the paint. The car is parked outside near the coast. The shop says the paint is defective and wants to claim against the supplier.
1. Identify the most likely causes and explain how inspection of the blisters distinguishes them.
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Read the blister contents
- Fluid with a rust-coloured pit at the base: corrosion starting at the substrate. The paint has lifted because the metal underneath is reacting, not because the topcoat failed.
- Clear or solvent-smelling fluid: solvent entrapment - coats applied too wet, too fast, or outside the overcoating window.
- Water droplets with no rust: moisture from the air line or from the substrate being sanded and left damp.
Most likely here The pattern points to moisture and preparation, not the paint: the drain has not been opened and the filter/regulator is 14 months past service, so water and oil aerosol are being delivered to the gun at 4 bar - a classic source of pinholes and blisters. Blisters concentrated over the filler suggest filler applied to bare metal or sanded-through areas with pinholes, then 1K products (permeable, not a sealer) under a 2K topcoat, on a car parked in coastal air. The rust-coloured pit confirms the corrosion started beneath the paint.
What is not supported A batch defect would normally show across panels and vehicles, not only over the filled area and the leading edge where stone chips and moisture collect.
2. Why should the shop not simply accept the 'defective paint' claim, and what evidence would you gather before any supplier claim?
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Why not The failure pattern follows the repair, not the product: confined to filled and repaired areas, with corrosion at the substrate, on a coastal car with an unserviced air line. A supplier claim made without evidence will be rejected and will also hide the process fault that will repeat on the next job.
Evidence to collect
- Photograph each blister, then probe and record whether the base is bare metal, filler, primer or rust.
- Measure dry film thickness across the panel against the product data sheet.
- Do an adhesion/cross-hatch or tape-pull test on an unaffected area and on the blistered area.
- Blotter or white-cloth test at the gun to show water or oil carry-over; log compressor dew point and filter condition.
- Check batch numbers, mix ratios, induction and pot life against the technical data sheets for every product used.
- Record the environment: 18 °C and 55% RH is acceptable, but panel temperature, flash-off times and film build are not documented.
- Check whether bare-metal areas received the specified epoxy primer and whether 1K etch primer is approved under the 2K system used.
If the evidence shows corrosion from inadequate substrate protection and moisture contamination, the claim is a process issue, not a product issue.
3. Set out the correct repair and the workshop changes needed so the fault does not recur.
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Repair
- Strip the affected area back to bare metal; do not simply sand and refill, because the corrosion is at the substrate.
- Remove all rust mechanically, treat the metal, and apply a 2K epoxy primer to bare metal and to any filled area - filler should never sit directly on unprotected steel.
- Apply filler only over cured epoxy, sand 180–240, then a 2K primer surfacer, sand 400–500 for metallic base.
- Seal with a compatible sealer before base coat; apply base to hiding with correct flash-off, then clear coat to the specified film build.
- Allow proper flash-off and drying before delivery, and protect the leading edge with the specified chip protection.
Workshop changes
- Service the filter/regulator, open the line drain daily, add a coalescing filter at the gun and check dew point; 4 bar at the gun is a symptom of restriction, not a setting.
- Write a job sheet with products, ratios, temperatures, flash-off and film build; keep technical data sheets on the wall.
- Train on the 1K/2K system rules: 1K products under 2K topcoats need a sealer, especially on a coastal vehicle.
- Inspect and sign off the repair before delivery, and keep a photographic record so a future claim can be defended.