What is filiform corrosion on a car?
Fine worm-like tracks creeping under the paint, lifting it, with no red rust, is filiform corrosion. It happens under paint on steel and on aluminium. On aluminium it shows as white powder in thin threads under the paint. It starts where the paint was broken, at a chip or an edge, and spreads sideways.
What kind of repair does it need?
Stripping the area back to clean metal, etch priming it for that metal, then painting. Aluminium and steel need different primers. The tracks run further under the paint than they look, so the stripped area is bigger than the visible damage.
Aluminium and steel need different preparation and primers. Ask the shop to confirm which metal the panel is before they quote.
What to do next
Ask the shop to confirm whether the panel is aluminium or steel before they quote. On aluminium, this is a paint problem, not a structural one. It still spreads, so it is cheaper to fix while it is small.
Sources
- Aluminum Associationindustry association
This form of corrosion appears as thin thread-like filaments filled with corrosion products (white powder) beneath the coating…
- Axaltapaint maker
phosphoric acid wash primer (etch-…
- PPG GBpaint maker
treat the surface with the correct metal treatment fluids and etch primer…
- GarageWeldingblog
Aluminum needs dedicated tools, contamination control, and a compatible joining process.
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