BillL
Well Known Member
Definitive Primer information
Primers are formulated for application to a wide range of materials, and, like most things, are better for some alloys than others.
I ran out of my old PPG, DP40, a zinc chromate based epoxy primer, so have been searching for something that is good for aerospace alloys.
There are two Mil specs that seem to apply to primers. One for water based (MIL-PRF-85582) and another for solvent based (MIL-PRF-23377K). You can Google these and scroll down to “everyspec” and download the documents. Many primers show up too.
A bit of searching yielded a DOD database for declassified military reports. Since the current trend was for elimination of ZnCr based primers, some testing of replacements was sought. One report found tested several different formulations on several aluminum alloys, and steel. Although it did not yield an straight answer, it did have a very good list of testing and pictures of the results. The name “Final Report on Alternative Chromate-Free Wash Primers “, ARL-TR-3932, September 2006 by Army Research Lab.
. The most interesting test results were the aluminum coupons. They scratched the coupon and exposed it to weathering and lab tests. In all testing the “control/baseline” was nearly perfect. It was a chromated primer meeting MIL-PRF – 23377K Type II, C2. That is strontium chromate, color green. Strontium chromate is 15-40%. Not just a little. While others were effective on other alloys, only the control was good on 2024-T3.
Here is a picture of some test coupons. You can google the report and read a detailed engineering report that will allow you to see how (some) engineers deal with data. Yawn.
Out of all if this I came to a couple of conclusions. I want a MIL-PRC-23377K primer for use under 2K topcoats to prevent lifting. For primer only (no top coat to be applied) applications like ribs etc, any ZnCr or other chromated primers will work. Check the MSDS, 1% is not going to work. Search on SkyGeek, for “zinc chromate primer” and lots of options appear. Even a rattle can (that can be wiped off with MEK so it won’t meet MIL-PRC-23377K but does meet TT-P-1757B specification with 85% ZnCr) would provide excellent protection if you don't want a top coat. It is a lacquer base.
Note: This is only about the efficacy of primers, not whether you should prime or not.
I could not find any testing on non-chromate primers that had testing on 2024-T3 compared to those that do. If you find a report and discover a primer, please post it, as it should be cheaper and more available.
Primers are formulated for application to a wide range of materials, and, like most things, are better for some alloys than others.
I ran out of my old PPG, DP40, a zinc chromate based epoxy primer, so have been searching for something that is good for aerospace alloys.
There are two Mil specs that seem to apply to primers. One for water based (MIL-PRF-85582) and another for solvent based (MIL-PRF-23377K). You can Google these and scroll down to “everyspec” and download the documents. Many primers show up too.
A bit of searching yielded a DOD database for declassified military reports. Since the current trend was for elimination of ZnCr based primers, some testing of replacements was sought. One report found tested several different formulations on several aluminum alloys, and steel. Although it did not yield an straight answer, it did have a very good list of testing and pictures of the results. The name “Final Report on Alternative Chromate-Free Wash Primers “, ARL-TR-3932, September 2006 by Army Research Lab.
. The most interesting test results were the aluminum coupons. They scratched the coupon and exposed it to weathering and lab tests. In all testing the “control/baseline” was nearly perfect. It was a chromated primer meeting MIL-PRF – 23377K Type II, C2. That is strontium chromate, color green. Strontium chromate is 15-40%. Not just a little. While others were effective on other alloys, only the control was good on 2024-T3.
Here is a picture of some test coupons. You can google the report and read a detailed engineering report that will allow you to see how (some) engineers deal with data. Yawn.
Out of all if this I came to a couple of conclusions. I want a MIL-PRC-23377K primer for use under 2K topcoats to prevent lifting. For primer only (no top coat to be applied) applications like ribs etc, any ZnCr or other chromated primers will work. Check the MSDS, 1% is not going to work. Search on SkyGeek, for “zinc chromate primer” and lots of options appear. Even a rattle can (that can be wiped off with MEK so it won’t meet MIL-PRC-23377K but does meet TT-P-1757B specification with 85% ZnCr) would provide excellent protection if you don't want a top coat. It is a lacquer base.
Note: This is only about the efficacy of primers, not whether you should prime or not.
I could not find any testing on non-chromate primers that had testing on 2024-T3 compared to those that do. If you find a report and discover a primer, please post it, as it should be cheaper and more available.
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