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Priming "mating" surfaces

mags591

Member
Hello,
I am curious to why if you chose to prime, many times people only prime the "mating" surfaces and don't prime the entire thing? Is there a benefit to it? I will be starting my build and live in Maryland right on the Chesapeake bay (W29).
I appreciate all the comments.
 
I've primed mating surfaces on skins etc. The logic I had was once those parts are riveted together, I can't easily access them again. I'm still building so it could be 10-20 years before I know if I was right or wrong.
 
i primed every surface inside the plane and wouldn't do it again. a lot of weight, cost and effort. when i look at some now 12 year old super shiny alclad surfaces which were not primed, i really start to question my strategy.

if i would do it again i would only prime the non-alclad parts and all the mating surfaces. an aircraft that doesn't live in some extreme environment should be good to go with non-primed alclad surfaces.

just make sure the alclad is clean, with no fingerprints, dirt and stuff before you rivet them together.
 
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100% Primed mine

I have been in the heavy jet overhaul business for 42 years, mostly as a structures specialist, dealing with corrosion control and aging aircraft extensively. I know most GA aircraft came unprimed, and I also as an IA keep up with a bunch of them too. Corrosion is no ones friend..especially in costal regions. When I built my -4 years ago, I didn't even question priming, as its in my blood to do it. I even primed the inside of every hole..yep. I use 2 part fluid resistant primer (Akzo and the likes), overthinned for extremely light weight penalty. Small parts are sprayed, inner skin surfaces rolled with a smooth foam cabinet roller. I can bet I added very little weight, and I know nothing will ever corrode. To me, I spent very little additional time, as I would prime large batches of prepared parts. All bolt holes were swabbed with a Q tip to cover the inside edges which is the root of all corrosion evil. Overkill maybe, but I couldn't sleep with out doing it this way.
 
The UN has been consulted.......

My take is this.

All internal surfaces are primed with a decent etch primer.

Anywhere there is a skin overlap, I Scotch the outside mating surface an inch or so from the edge and quickly run a rattle can etch down it and try cleco up asap. That way, the two surfaces will tend to soften together and help the condensation from starting tinworm later on.
 
while I primed all parts, I believe the reason that mating surfaces are more susceptible to corrosion is that the mating areas will tend to hold moisture longer than exposed surfaces, due to reduced air contact. You can experiment. Take a piece of mild steel and place a second piece of steel half its size on top of it. Pour some water on it and come back the next day and observe. I have observed this several times with steel in my projects.
 
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For those who wish to do so

Someone mentioned Q-tips. If you are going to use primer inside of fastener holes (some applications call for such like Hi-Loks, depending on material), finger nail polish bottles work great for single part primers. If using a high solids primer, find said bottle that still has the mixing BBs in it. A thought for those who wish to do so. I do between dissimilar metal applications. Here in humid AF Florida, the air is full of catalyst to promote all types of corrosion.

Different subject = fog, fog, fog your airframe.
 
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Primer method

I have been in the heavy jet overhaul business for 42 years, mostly as a structures specialist, dealing with corrosion control and aging aircraft extensively. I know most GA aircraft came unprimed, and I also as an IA keep up with a bunch of them too. Corrosion is no ones friend..especially in costal regions. When I built my -4 years ago, I didn't even question priming, as its in my blood to do it. I even primed the inside of every hole..yep. I use 2 part fluid resistant primer (Akzo and the likes), overthinned for extremely light weight penalty. Small parts are sprayed, inner skin surfaces rolled with a smooth foam cabinet roller. I can bet I added very little weight, and I know nothing will ever corrode. To me, I spent very little additional time, as I would prime large batches of prepared parts. All bolt holes were swabbed with a Q tip to cover the inside edges which is the root of all corrosion evil. Overkill maybe, but I couldn't sleep with out doing it this way.

That's a very interesting process. Thinned Akzo applied with a roller. I bet it's well protected with a very thin layer and minimal labor. You might consider writing that up to the Tips thread.

I sprayed P60G2. Very thin. I calculated the weight but it was so small the number didn't make sense. Less than a pound. All surfaces plus any overlapping exterior fay services.
 
After going through a build, I ended up in the "I primed everything but would not on my next build" army in the Primer Wars. Not just the extra weight and cost, I found that it measurably extended the build time, at least the way I did it. Instead of:

Drill, debur, dimple, rivet part A, drill, debur, dimple, rivet part B, drill, debur, dimple, rivet part C​

... you end up with:

Drill, debur, dimple part A, drill, debur, dimple part B, drill, debur, dimple part C, then clean all parts, wait for a sunny day, scuff, etch all parts, wait while they dry, spray all parts, wait while they dry, then rivet everything together.​

You blow at least an extra day every time there's a batch of parts waiting to prime, and you end up having to go out-of-order on the plans in quite a few places, potentially leading to mistakes if you're not careful.
 
It is your plane, do whatever you like to do with it.

I'm in this camp.

There's so many variables too, that there's no one correct answer. In an extremely dry climate and only fly locally will need significantly less corrosion protection than a heavy jet that flies from the land of snow and deice fluid to Florida's salt air.
 
I'd say resale was one major factor in my decision. If I decide to sell after building and flying for a while, why exclude those in the pro primer camp, or those that live in coastal areas, from considering my plane?
 
Hello,
I am curious to why if you chose to prime, many times people only prime the "mating" surfaces and don't prime the entire thing? Is there a benefit to it? I will be starting my build and live in Maryland right on the Chesapeake bay (W29).
I appreciate all the comments.

In the Navy we "fay-sealed" ribs and lap joints in order to prevent moisture intrusion, particularly in large faying surfaces, such as longeron/skin unions, lap joints at compound curves and anywhere large extrusions or weldments were joined. I suspect the idea of priming just the mating surfaces falls along the same line of reasoning.

In fay-sealing, we used Mil-S-8802 - what we commonly call ProSeal. So, it added a bit of weight. No way I'd want to do this to a whole Van's kit. Just fay-sealing the fuel tanks is enough of that stuff for me.

I primed everything with SEM gray primer from the rattle can. Least amount of hassle to apply. All the bellcranks and push-pull tubes are gloss white so they stand out against the light gray non-reflective finish. I just find this easier to look at when the need arises (than shinning a flashlight down a shiny unfinished tail cone). Interior wear areas like cockpit floors, armrests and panels get a couple of coats of Eastwood 2k Aerospray matt clear over the primer gray for durability.
 
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Our rv8 is a little over 20 years old. I had to reskin the rudder a few a years ago and while the inside was clean I found corrosion underneath the stiffeners when I removed them. The airplane has always lived in a hanger in Wisconsin.
 
Our rv8 is a little over 20 years old. I had to reskin the rudder a few a years ago and while the inside was clean I found corrosion underneath the stiffeners when I removed them. The airplane has always lived in a hanger in Wisconsin.

Are you saying they did or didn't have primer on the surfaces?
 
Before I built my plane I had the (good?) fortune to do a fair bit of owner maintenance on some older Cessnas, and having seen what can happen over time to bare AL or even primed but scuffed (especially in nooks and crannies that are hard or impossible to get to for cleaning / treating), there was no question for me, I knew I would be priming my RV's interior surfaces.

Quite possibly an alclad-only plane kept hangared in some parts of the country would never see corrosion in our lifetime. Nevertheless I'd rather be safe than sorry. Having a good coat of primer on interior surfaces gives me that peace of mind.
 
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Below is a photo of the interior of my nearly 60-year old 170 wing in 2014 after being opened for inspection courtesy of a hand-propped Cherokee. It spent the first two years of its life in Marianna FL, the next 26 years OUTDOORS at Stone Mountain Georgia (east of Atlanta), at which point I bought it. Then there was 8 years in shade and unheated hangars at Tullahoma TN, 6 years in an unheated hangar in Picayune, MS (near the Gulf Coast), and almost 18 years in Tehachapi, CA, a couple of which were outdoors. On rebuild, there were NO parts that would have required replacement for corrosion.

On my RV-7, I’m priming primarily for resale, but (mostly) only faying surfaces of sheet parts, as well as all 6061-T6 parts per the build manual. My reasoning is that all other surfaces can be inspected, and, although the designation is different, it is the same alclad alloy from which my 170 was built. My process would be the same regardless of where I lived; it makes no sense to me to build a 200 mph airplane for only one climate.
 

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