Blisters and Sandscratches on a Refinished Door
A door is refinished after a dent repair. The technician applied polyester body filler directly over bare steel that had been wiped with thinner, sanded the filler with 80-grit and then 180-grit, applied a 2K primer, and wet-sanded the primer with 600-grit before basecoat. The basecoat was applied in two coats with a 5-minute flash, followed by two heavy clearcoat coats 8 minutes apart. Shop temperature was 18 degrees C and relative humidity 70 percent; the compressor has no working dryer and the water trap was not drained that morning. Ten days later the customer returns. There are blisters along the lower door edge, fine scratches visible in the clearcoat on the upper door, and small crater-like pin holes on the top edge of the door. The filler edge near the door handle also shows tiny pinholes. The technician wants to polish the door and re-clear the whole panel.
1. Diagnose each defect and explain the most likely cause, using the evidence in the setup.
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The blisters along the lower edge are most likely moisture or contamination trapped under the coating. The undrained water trap and missing compressor dryer are strong evidence, and the lower edge is where water and solvent can collect. The fine scratches on the upper door are sanding scratches that were not removed before topcoat: 80-grit followed by 180-grit is too coarse for the subsequent primer and 600-grit wet sanding to hide, especially if the primer was applied over deep scratches. The crater-like pin holes on the top edge are solvent pop, caused by heavy clearcoat coats applied only 8 minutes apart at 18 degrees C, which traps solvent under a skinned surface. The tiny pinholes at the filler edge are filler pinholes from poor mixing or spreading, not the same as solvent pop. Polishing and re-clearing would trap the moisture and scratches and would not correct the filler pinholes.
2. State the correct rework procedure and the material or process changes needed.
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The affected area should be stripped rather than patched over. Remove the clearcoat and basecoat, then remove the filler if blistering or adhesion loss reaches the substrate. Sand the bare steel to a clean, feathered edge, treat it with the maker's epoxy primer, and apply filler only over cured epoxy, not directly over bare steel. Mix filler accurately, spread it firmly to avoid pinholes, and sand with a proper progression, for example 80-grit to shape, then 180-grit and 240-grit before primer. Apply primer in suitable coats, use a guide coat, and block with 400-grit then 600-grit wet or dry as specified. Clean with wax and grease remover and dry with clean, dry air. Repair the compressor dryer and drain the trap before spraying. Apply basecoat and clearcoat in thinner coats with the correct flash time for the shop temperature, then force-dry or allow full cure before polishing.
3. What quality controls and customer communication should follow the rework?
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Before spraying, the technician should spray a test panel with the same materials and check for solvent pop, pinholes and moisture contamination. The air line should be tested for water and oil, the gun should be cleaned and set correctly, and the shop temperature and humidity should be recorded. After repair, the door should be inspected under bright light for scratches, pinholes and blisters, and the repair area should be checked for adhesion. The job card should record the original defects, the strip-and-repair method, the materials and batches used, and the curing time. The customer should be told honestly that the first finish failed because of moisture and surface preparation, that the door has been stripped and refinished, and what care is needed during the paint cure. A warranty on the rework should be stated in writing, and the customer should be advised to return if any blister or scratch reappears.