Filler Blisters After a Rainy-Season Door Repair
In the rainy season in Kumasi, a five-year-old sedan is in for a dent on the right rear door. A technician repairs the dent with polyester body filler. Before filling, he scuffs the factory finish with P80 paper, blows the area with shop compressed air, and wipes it with a rag soaked in thinner. The shop's compressed-air line has no dryer or filter, and the drain valve on the receiver has not been opened for months. Relative humidity in the workshop is 85%. The filler is mixed 'by eye' with about 3% hardener instead of the specified 2%. After sanding, the technician applies a primer-surfacer and sends the car out. Three weeks later the customer returns: six blisters, 5-10 mm across, have lifted on the repaired area, and each contains a small amount of brown liquid. The paint around the blisters is intact.
1. What are the most likely causes of the blisters? Explain the mechanism rather than just listing possible causes.
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The pattern points mainly to moisture, not simply 'bad filler'. The air line has no dryer or filter and the receiver has not been drained, so water and oil aerosols are carried to the panel. At 85% RH the primer and filler cannot flash cleanly, and the thinner-soaked rag does not remove water-soluble contamination and may leave solvent trapped. Mixing about 3% hardener instead of 2% makes the filler over-catalysed and more porous; pinholes then let moisture through. Blisters 5–10 mm across with brown liquid after three weeks are consistent with corrosion at the steel/filler interface, not the small, quick solvent pops you would expect from trapped solvent alone. The fact that the surrounding paint is intact supports a local substrate/moisture problem. Do not just grind and refill; the source must be corrected.
2. Outline the correct preparation and application sequence for this repair, including checks before priming.
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Strip the failed filler back to bare metal and inspect for rust; remove all contamination. Drain the compressor receiver, fit a coalescing filter/dryer, and test the air with a blotter test before use. Clean with a proper wax-and-grease remover using the two-cloth method, not thinner. Sand the area to P80–P180 with feathered edges, apply epoxy primer to bare steel, then polyester filler per the data sheet - mix by weight at 2% hardener, spread thin, and avoid pinholes. Sand P80 then P180, apply primer-surfacer, guide coat and block flat. Check film build and humidity; use a hygrometer. Seal and topcoat only after the primer is fully cured. If the manufacturer's system allows filler over sanded OEM paint, follow it; in this case bare metal/epoxy is the safer choice.
3. How would you confirm the cause before refinishing, and what would you tell the customer?
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Before refinishing, remove one blister and inspect its underside for moisture, rust and filler porosity; check whether the steel is pitted. Run a blotter test on the air line and check the compressor drain, then review the filler mixing ratio and the workshop humidity record. If the same failure appears on a test panel applied with dried air, the problem is product or mixing; if it does not, the air line is the likely source. Tell the customer plainly what failed, show the corrosion, explain that the repair will be redone to bare metal with dry air and correct priming, and give a realistic warranty. Do not blame the customer's use of the car.