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Ask the Expert Question-and-Answer Archive

by Ron Joseph

January, 2005

Acid Etch Wash Primer and Alodine or Chemfilm

Q. I am currently building a kit airplane, from it is mostly aluminum. I have been told by some aircraft builders to use mital prep. 79 rinse with water, then alodine which I did. Vans says to use a Sherwin Williams industrial wash coat, which I did, on the inside of the parts only. I read Sherwin's directions and they say to not use over alodine, any thoughts? Also can you tell me what paint you would use on your plane?

A. Alodine, Chemfilm and other brands of conversion coatings for aluminum are intended to passivate the aluminum substrate. They form a tenacious, inert film on the substrate. Therefore, you do should not apply a wash primer over the conversion coating, because excess acid in the primer cannot react with the aluminum substrate. If you do apply the wash primer the excess acid will react with your epoxy primer and that will degrade you coating system.

I suggest that you follow Sherwin-Williams' good advice.

Best Wishes,

Ron Joseph

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