Prep and Prime Before or After Dimpling

[YoVuelo]

This may be a non-issue, but I'd like to know what the "consensus" of thought is on the order of dimpling and priming. I've defaulted to 1)deburring, 2)prepping the parts for priming, 3)priming, then 4)dimpling. Seems to me that the part scrubbing and prepping prior to priming (I'm using Stewart Ecoprime) is easier done on a non-dimpled surface. The adhesion of the primer is good and I've never had any flaking or other damage to it from the dimpling process. So, am I worrying about nothing or is there something that I haven't considered...?

[D-Dubya]

There are ongoing epic wars over primers, the need for priming, and priming techniques. Your approach is the way many of us have chosen to do it. Primer, properly applied, will typically not flake off during dimpling. And it's much easier to prep and prime an un-dimpled surface. You're spot on.

[bruceh]

Much easier to prep prior to dimpling. You may have to let the primer sit for a day or two. I found that with the AKZO epoxy primer it was solid and impervious after a couple of hours, but the NAPA rattle can needed at least 24 hours to set up enough to handle the dimpling process unscathed.

[Kyle Boatright]

I think you'll get better adhesion if you clean/prep the material, dimple, then prime it. The C-frame dimpler is pretty tough on things, even primer that usually sticks pretty well.

[BillL]

I just wish there was data available about the structural effect of primer under a rivet head. In the industrial world, paint under a high pressure structural joint is not acceptable. Think connecting rod caps, transmission interface, final drive housings. I did not have the opportunity to do my skins, but on components, the order was skuff, dimple, degrease, alodine, rivet, prime. This is probably (definitely) impractical with skins and larger pieces, and may not make any difference at all. Few primers are effective with 2024 (Cu alloys), and using a non-effective primer has no downside if primers aren't needed anyway, but if it is put under critical rivet heads, there is a possible downside for no corrosion protection. But - irrelevant without some real data.

[TCONROY]

My technique was to prep the part for priming (scuff & wash), dimple, prime. There isn't a right or wrong way, just do what works best for you.

[rvbuilder2002]

Originally Posted by BillL

I just wish there was data available about the structural effect of primer under a rivet head. In the industrial world, paint under a high pressure structural joint is not acceptable. Think connecting rod caps, transmission interface, final drive housings. I did not have the opportunity to do my skins, but on components, the order was skuff, dimple, degrease, alodine, rivet, prime. This is probably (definitely) impractical with skins and larger pieces, and may not make any difference at all. Few primers are effective with 2024 (Cu alloys), and using a non-effective primer has no downside if primers aren't needed anyway, but if it is put under critical rivet heads, there is a possible downside for no corrosion protection. But - irrelevant without some real data.

I fully understand your logic Bill, but if it were valid, there would also be problems with primer on any portion of the parts because it would also be on the mating surfaces of the dimples between skins and sub-structure.
Service history has shown that not to be a problem. Regardless, most RV builders don't primer the exterior surface of the skins so there usually isn't any primer under rivet heads. The few that do primer exterior surfaces are wasting the money and effort because it will have to be re-primed again when the assembly finally gets painted, because most primers we use now a days have a very short time period they must be top coated in to get good cross link adhesion with the paint. Primer more than a couple days old usually has to be sanded and the re-coated before top coating.

I do have some real data regarding corrosion protection gained by by using a primer coating that even the manufacturer doesn't make claims for , unless it is top coated. I have a test sample of Sherwin Williams P60-G2 that has been hanging outdoors for ~14 years. It shows strong evidence that the area of the sample covered with primer is definitely protected (when compared to the area of the sample that was left bare).

[BillL]

Originally Posted by rvbuilder2002

I fully understand your logic Bill, but if it were valid, there would also be problems with primer on any portion of the parts because it would also be on the mating surfaces of the dimples between skins and sub-structure.
Service history has shown that not to be a problem. Regardless, most RV builders don't primer the exterior surface of the skins so there usually isn't any primer under rivet heads. The few that do primer exterior surfaces are wasting the money and effort because it will have to be re-primed again when the assembly finally gets painted, because most primers we use now a days have a very short time period they must be top coated in to get good cross link adhesion with the paint. Primer more than a couple days old usually has to be sanded and the re-coated before top coating.

I do have some real data regarding corrosion protection gained by by using a primer coating that even the manufacturer doesn't make claims for , unless it is top coated. I have a test sample of Sherwin Williams P60-G2 that has been hanging outdoors for ~14 years. It shows strong evidence that the area of the sample covered with primer is definitely protected (when compared to the area of the sample that was left bare).

Thanks for the real world information on the structural aspect of a pre primed , dimpled, rivet joint. That was my real question, and certainly quantifies the issue. Further study on the joints, it appears military has been using a wet assembly of riveted joints for many years, that make sense as it would be a minimal thickness of timer and not restrain setting contact. It is limited to chromated primers, though along with alodine pretreatment. At least from research on DoD tech databases so far. Even if fatigue life were reduced from a dry assembly in DoD tests, this real world evaluation is proof enough for our RV application.

I said most primers don't work (for 2024), not to include chromate primers. The one you mention is contains chromates and would be expected to provide the best protection with or without topcoat for 2024 alloy. Ref http://www.dtic.mil/dtic/tr/fulltext/u2/a464961.pdf

It is always good to hear about your experience with these things. Thanks

[YoVuelo]

Thanks to all for the insights. What got me thinking about this was a conversation with my brother-in-law, who works for Boeing, and was asking me if we wet set rivets like they did. I said no, we really wern't worried about a 50-60 year airframe life like the military. I do want to ensure a decent airframe life and integrity in whatever conditions this aircraft may encounter, so I have elected to prime the interior parts and the interior faying surfaces of the skins. However, it did get me to thinking about the process and what made sense.
Bill, I had wondered a bit about the integrity of the joint/rivet integrity with the layer of primer between the head and the faying surfaces, etc. but Scott seems to have allayed that concern. As a side note, I do all the mechanical counter sinks prior to priming, so all of my spars and what not are completely coated. Thanks again to all of the people and for the knowledge that this form seems to have at its fingertips!

[wirejock]

Originally Posted by rvbuilder2002

I fully understand your logic Bill, but if it were valid, there would also be problems with primer on any portion of the parts because it would also be on the mating surfaces of the dimples between skins and sub-structure.
Service history has shown that not to be a problem. Regardless, most RV builders don't primer the exterior surface of the skins so there usually isn't any primer under rivet heads. The few that do primer exterior surfaces are wasting the money and effort because it will have to be re-primed again when the assembly finally gets painted, because most primers we use now a days have a very short time period they must be top coated in to get good cross link adhesion with the paint. Primer more than a couple days old usually has to be sanded and the re-coated before top coating.

I do have some real data regarding corrosion protection gained by by using a primer coating that even the manufacturer doesn't make claims for , unless it is top coated. I have a test sample of Sherwin Williams P60-G2 that has been hanging outdoors for ~14 years. It shows strong evidence that the area of the sample covered with primer is definitely protected (when compared to the area of the sample that was left bare).

Scott
My bird is fully protected inside with P60-G2. I've heard about the sample and would like to see it. it be possible to post or e-mail a photo of it?