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Accelerated corrosion in damaged car parts

Isn't it true that a body part of a car that has been hit leaving a crease in the metal and exposing bare metal on both the front and backside, if Bondo-ed ⇦ this on eBay or Amazon [affil links] and painted on one surface and the creases and exposed metal that is on the other side is not touched, will then start a corrosive reaction, and possibly at an accelerated rate possibly because of filiform corrosion, crevice corrosion and because of the principle of corrosion cells and stress cells. Thank you for your help in explaining this reaction.

J.W. [last name deleted for privacy by Editor]
student - Louisville, Kentucky
2004

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You're making it sound more complicated than I thought it was, J.W. If bare metal is exposed, it will rust. If the area is tensilely stressed because of the crease, that area will be anodic to the rest of the panel and will rust preferentially. Part of the reason for phosphate pretreatment before painting is to discourage filiform corrosion; if the phosphate was scratched off in the accident, you no longer have that protection.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

Ted could I ask what your credentials are.

J.W.
student - Louisville, Kentucky
2004

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I apologize, J.W., but no. This is a public forum for technical assistance and camaraderie, and where ideas are tossed out there for consideration. We don't want the posters' credentials to become part of the process because it spoils the camaraderie if someone is challenged. And no one wants to be quoted without their knowledge and agreement. If this is a school project where you, very properly, want to quote an authority, please see if your school or town library can get you Samuel Spring's Preparation of Metal for Painting ⇦[this on on Amazon affil links] or Rausch's "Phosphating of Metals" ⇦[this on Amazon or AbeBooks affil links]. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004