Ask the Expert Question-and-Answer Archive

by Ron Joseph

November, 2008

CARC Paint

Q. I am working with 4130crs and 6061-t651 plate. We are using dry abrasive blasting with the proper type and grit for each metal per bac5748.Pretreating is next. On the 4130crs they are calling for TT-C-490 (Zn phosphate). In the same block they include DOD-P-15328 (WASH PRIMER) and MIL-C-8514 (WASH PRIMER).Do I use the wash primer over the conversion(Zn phosphate)? Now to the aluminum. There call out is for MIL-C-5541 (Chromate conversion).In the same block of information they include DOD-P-15328 (WASH PRIMER) and (MIL-C-8514(WASH PRIMER) and MIL-A-8625(ANODIZE). We don't need to anodize but do we need a wash primer over the MIL-C-5541(Chromate conversion)? We also have another primer MIL-P-53022,53030,53084 for the steel and MIL-PRF-23377, 85582 for the aluminun to go on next followed by a Topcoat MIL-C-53039. After sandblasting the aluminum, it has to be solvent cleaned and alkaline cleaned before any coatings. Thanks

A. Here is a simple reply to your questions. If you apply TT-C-490 Type I, zinc phosphate on the CRS, you do not need to, nor should you apply the wash primer. In fact, if you apply wash primer over the zinc phosphate you can expect blistering. Similarly, if you apply MIL-C-5541 Class 1 A, chromate conversion coating over the 6061-T651 aluminum, you need not and should not also apply the wash primer. Again, you can expect blistering if you also apply a wash primer over the conversion coating. You can and should apply the primers, MIL-P-23377, MIL-P-85582, MIL-P-53022 or MIL-P-53030 directly over the respective pretreatments.

Regards,

Ron Joseph