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Q&As since 1989


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No acid wash primer over chromate

One of my clients is required by his customer, which is a very well known aircraft oriented conglomerate, to acid wash prime over aluminum chromate. I have counseled him that acid wash primers degrade the chromate and that is common to do one or the other but never to do both on the same metal. I can come up with many vocal supporting people but so far I cannot "document" this advice. Help!

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2002

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First of two simultaneous responses -- 2002

Carl Izzo, author of the Painting Clinic column in Products Finishing magazine, dealt with this subject in 9/98 and 5/99 columns. The upshot: he does not recommend using phosphoric acid activated wash primers over pretreated metal, including chromate conversion coatings. "And yes I know that their uses are spelled out in Military Specifications over zinc chromates. However my answer still stands." - 5/99 column.

For what it's worth...

Michel Moninger
- Sarasota, Florida

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Second of two simultaneous responses --

If your customer - the finisher - is working without a Specification and QPL, their customer is crazy! There should be a rigorously controlled process that is well defined and unambiguous. (We agree, however, with your premise one or the other, not both...)

Megan Pellenz
- Syracuse, New York
2002

I also have a call out for a wash primer (DOD-P-15328 or MIL-C-8514 [from DLA] ) over TT-C-490 [from DLA] , I (Zn phosphate) for steel and (DOD-P-15328 or MIL-C-8514) wash primer over MIL-C-5541 (chromate conversion) for aluminum. Is this correct or do you use one or the other, conversion or wash primer? They are not very clear on their call out!

Gary M Swaboda
Aerospace - Orangeburg, South Carolina
November 26, 2008

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July 14, 2009

I went through this exact issue during the 80s, and if I remember correctly, Mr. Izzo responded in FINISHING MAGAZINE. (I'm thinking January or February, around 1986 or 87.) Bottom line, you can increase the adhesion of the subsequent coatings on treated aluminum by eliminating the wash primer. It's intended to be used when you CAN'T use chem film, a.k.a. Iridite. I even won a $10,000 dollar award for this suggestion at the company I worked for at the time. I'm POSITIVE you don't want that stuff (wash primer) on chemically treated aluminum. BAD NEWS !
Take care.
Rick

Richard Yorke
- Miramar, Florida, USA