Corrosion of small/missed spots/edges that are not primed

I'm priming, so no need to get into that conversation

Being an new builder that chose to prime, I have of course been over zealous making sure that everything is covered in primer. Any time I find a small miss here or there, or on the edge of a part where I missed; I end up going back and fixing it. This is tedious as heck with AKZO (scuff around area, respray or touch up etc), especially finding it the next day/s.

So question, the "edges" of all of our parts are not covered in alcad by definition. How does corrosion effect parts like this? Does only the exposed metal corrode, or can it lead to corrosion getting "under" the alcad or primer.

If corrosion can't get "under" anything, I'm inclined to believe that any tiny misses in primer here and there won't cause any problems.

Has anyone every seen corrosion on a small spot of missed primer, or on an edge?

DoD test papers show 2024 is the most prone to corrosion (copper alloys) and attention to the edges is addressing that. However, if you are priming with AKZO, it highly likely is the formulation with strontium chromate (replacement for zinc chromate) and acts as a sacrificial element. Extreme scratch tests in salt sprays and outdoors show small areas are effectively neutralized for the duration of testing. It is very effective in slowing growth.

So as a practical matter, doing what you can is good, nothing in areas is not a killer. After all, the most complete way is to wet prime the rivets before setting, and you will not be doing that. Nor spending life on a carrier deck in salt mist for 40 yrs.

Not saying you should do anything extra, I am inclined to agree with you.

Edges are most important..

On Alclad sheets, the edges (including holes) are the most important areas for protection. That is where 99% of corrosion will start. Think of it a sheet of plywood left in the rain..the edges will be the first to go. I personally go as far as "swabbing" every hole with primer before riveting if it wasn't covered when I primed, but that attention to detail comes from my day job. The Alclad coating makes the skin sheets polish friendly, and will withstand nearly anything for years, but the edges are prone to corrosion. Take a look at some airliners over the last several years..you will find polished Boeings, but not any polished Airbuses..they don't build with Alclad, and must remain fully painted.

fixnflyguy said:

On Alclad sheets, the edges (including holes) are the most important areas for protection. That is where 99% of corrosion will start. Think of it a sheet of plywood left in the rain..the edges will be the first to go. I personally go as far as "swabbing" every hole with primer before riveting if it wasn't covered when I primed, but that attention to detail comes from my day job. The Alclad coating makes the skin sheets polish friendly, and will withstand nearly anything for years, but the edges are prone to corrosion. Take a look at some airliners over the last several years..you will find polished Boeings, but not any polished Airbuses..they don't build with Alclad, and must remain fully painted.

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I will agree with my friend, Bill, who is the expert on this subject! I can tell you that living here in Florida with my RV-6, in every case of corrosion I have encountered on my 17-year-old airplane, it has started from the edge of a skin. And my airplane was primed inside and out with an epoxy primer, DeSoto, as I recall. I haven't had a lot of corrosion, maybe a half dozen instances where it was bad enough, in my opinion, to treat it.

Thanks so far for the info. So many Bills, haha.

Out of curiousity, what are your's (or any of the readers) thoughts on just using a rattle can, strontium chromate, etching, primer for touch ups on top of already cured AKZO?

Is scrubbing worth it for tiny touch ups, or just use the rattle can?

Can the etching part do any harm to the already cured AKZO?

Besides the small "misses" here and there, this also could be a nice way for any scratchs that happen due to bucking etc on a assembly (yes, masking tape is your friend, I know ).

I know rattle can isn't nearly as burly as AKZO, so assume its in a location where "mechanical" protection isn't a primary concern.

Touch ups

You can call me Bill for now.
I use rattle can NAPA 7220 for the little touch ups. I spray a little in a disposable cup and use a Q-tip to apply it.
No need to scuff the Akzo. That stuff is hard. Just touch up bare spots. Etching primer wont harm the Akzo.

I went overboard with mine.

Alumiprep, alodyne, and them SEM rattle can primer. 11 years, do far, so good.