To prime or not to prime (or what to prime), that is the question.

I've read every post I can find regarding priming, but I'm still conflicted. Opinions seem to range from "prime every surface" to "prime only what's absolutely necessary." Even regarding mating surfaces with dissimilar metals (galvanic corrosion), there's no concensus.

At one point I found an article (which I can no longer find -- I have a hard time navigating the official site) on Van's site espousing the minimalist approch, pointing out the lack of primer on the interior of Cessnas, Pipers and such.

If some long-time builder could point this newbie in the direction of a definitive article on this, I'd appreciate it.

Jerre

Only a chemist knows the answer and even she might answer, "It depends."
Joe Gores

I primed the 9A I'm building (using the NAPA self-etching primer). If I do another plane, I doubt that I prime it. At most I'd do the few parts that are not alclad and the ferrous parts.

Having said this, you are not going to get a definitive answer because the decision is subjective. Corrosion happens slowly. It's not going to turn your plane to a white powder while you're flying. It will corrode faster near salt water than it will inland. If you see corrosion there are products to treat it. Odds are it won't be needed. That's my subjective opinion after owning a 1974 Cherokee that's unprimed (mostly) for better than 10 years.

I feel your pain. I am another first time builder that did the same frustrating research that you did. What I decided was that if the interior parts of 50-year-old Pipers and Cessnas were not primed and had no issues, that is pretty compelling.

I looked at a BUNCH of builder logs. I have great respect for the total-primers - who mix 2-part primers and use spray guns (and then have to clean them after every use). To do that requires a lot of time, and also a lot of planning and construction delay to have reasonably sized batches. I did not see that kind of effort to be needed. To each his own, you are building the plane YOU want!

I found rattle-can self-etching aluminum primer to be a good choice. About $6 at Home Depot, Marhyde was about $15 a can at O'Reilly Auto.

I decided to prime the parts (scuff with scotchbright pad first) that were associated with the bottom of the plane. That included the inside of the bottom skin but not the fuselage ribs. Although I did prime the tailcone ribs, they were easy. I primed all small parts that were exposed and not fully enclosed - things like the rear spar of the vertical stabilizer, hinges, etc. Obviously, the tail's rear bulkhead. If it was small, connected, and easy to do I primed it. Wing ribs and interior stuff like that? No. I was probably not totally consistent in my choices!!!

Although, it turned out later that some of these choices would be primed again in the painting process. Sometimes that is tough to visualize early in the building process. (For my full painting description, see this thread:
http://www.vansairforce.com/community/showthread.php?t=67760

For some photos (not a builders log, just some quick sequential photos of the build which does show some painting info - easy to go through in 5 or 10 minutes) see here:
http://www.facebook.com/media/set/?set=a.151245628228677.26907.100000297210697&type=1&l=a2b29e894a

Hope this is useful. This is a long - standing controversy and your final opinion will be as good as anyone elses!

And if anyone is a fan of sci-fi and particularly robots, see these pics I took at The Robot Hut!
http://www.facebook.com/media/set/?set=a.214137035272869.62811.100000297210697&type=1&l=87a25084c8

AeroDog said:

I've read every post I can find regarding priming, but I'm still conflicted. Opinions seem to range from "prime every surface" to "prime only what's absolutely necessary." Even regarding mating surfaces with dissimilar metals (galvanic corrosion), there's no concensus.

At one point I found an article (which I can no longer find -- I have a hard time navigating the official site) on Van's site espousing the minimalist approch, pointing out the lack of primer on the interior of Cessnas, Pipers and such.

If some long-time builder could point this newbie in the direction of a definitive article on this, I'd appreciate it.

Jerre

Click to expand...

In fifteen years of being part of the RV community, I have never seen a definitive article on this subject. I doubt one has been authored.

My personal thoughts about priming circa 1997 are here. I've found no reason to change them since then.

prime the non al clad and get on to building

I primed everything. Cost was about $500 (2 part epoxy, HVLP, supplies, etc.)
I didn't want to miss out on the experience

Now that I've done it, if I were to build again I would skip priming.
It does add time to the build (not a big deal for a slow build), it does add weight, and it isn't necessarily environmentally friendly. Like others have said, the alclad provides plenty of corrosion resistance and any problems will be years/decades down the line.

I built a BD-4 (all alum) and sold it to pay for the -8. Ended up reducing the price substantially because of discovered corrosion on several wing ribs and skins which required significant repair, not to mention the questions about other less easy to inspect areas all potential buyers asked about.

Because of this experience I chose to prime EVERTHING (2 part epoxy) and was still able to complete the slow build (including prime and paint) in just over 21 months start to first flight.

YMMV

My mostly unpainted and unprimed 1955 Cessna has little corrosion. The catch is that it's always been an inland airplane, and all the corrosion it has came from being tied down outside for six months near Philadelphia.

Even hangared for nearly three years in Oxnard, CA (like three miles from the ocean) didn't do much damage. Those few months outside in PA sure did, though.

I suppose the moral is that if your plane is ever going to be kept within a hundred miles of a beach, prime it. Otherwise, don't. And if you do, try to be rigorous about saving weight elsewhere to compensate.

Incidentally, 6061-T6, although not alclad, has good resistance to corrosion. Some people don't prime that. In marine use, though, it's either anodized or painted or both. But that's in marine usage.

Dave

Me too

I am not disputing anything that anyone has written about not priming but I am leaning towards 'priming everything' inside. I have see the insides of a few of the C-152, C-172, PA-161s, etc which are based at a flying club a few miles away in Long Beach - they all have corrosion to one extent or another.

Wondering about seaplanes, etc - are they primed inside?

I live about 1/8 mile from the Pacific Ocean and gotta tell you, some types of bare steel begin to rust here in about a week. So, I think that priming everything inside might be a good idea.

Given the costs and waiting period of a hanger in these parts, my RV-12 will have to be left outside - OR, take it home after flying and put it away in the garage OR, I can base it in a hanger inland about one hour drive away.

Bringing it home on a trailer is done by so few (none?) of you that I am pretty sure that it isn't practical.

I own two cases of NAPA xxxx spray cans. Anyone want to buy them? Think that I'll switch to a two part primer/sealer and spray it on with an hvlp gun.

It does depend

Mich48041 said:

Only a chemist knows the answer and even she might answer, "It depends."
Joe Gores

Click to expand...

I used to be a chemist, an analytical industrial chemist for Ferro Corp. (a US company actually)

so Joe your right the answer is "depends." (and I am only a she with a little wine and right set fo high heals)

What it depends on, and I insist that you count this as the official answer.

It depends on the thickness of the the Pure Aluminium layer on the outside of the sheeting, balanced by the corrosive forces the Aircraft experiences.

Protection
There is a layer of protection. It is Al2O3. This is EXTREMELY unreactive. It is what happens to pure aluminium when exposed to air. Not alot can react with it. So even a couple of nanometers of this will do.

Just right for it to go wrong
However if you scratch it, you expose the strength baring alloy beneath. No surface is perfect. Under a microscope it has natural variation. If at some small point you have a low point in the thickness of that protection, and that gets scratched then you will get corrosion from salts, organics, all that stuff. This is not a big deal and happens everywhere. It appears as a little pin prick of corrosion. If there is a high degree of salt exposure in an area where it stays wet to create the ion bridge that makes the whole thing work then that pin prick will continue to grow and will not stop untill

1. The water goes away
2. It hits something it can't react with any more (like the end of the part, ie its all gone)

So what do you do?
Do you know the level of integrity of the outer layer of Al2O3. That is the call that Cessna and Piper made. They also made this call. How long will it be before the thing coroded to a point of it being a probelm structurally. They made that call. I mean real maths, statistics, lots calculations, experiments, guys in white coats and clip boards ( I used to be one fo them ) then they looked at legal liability and the issue with reducing brand reputation if they get an issue. They then looked at how much it would cost to prime. ie Risk vs Cost

They found that with the quality of their Aluminium sheeting, with the mission of the aircraft, with the degree of corrosion, it would mean they could run the risk, considering how much it costs to prime, it was economically effective to not prime.

Knowing all of this, what am I doing
I am planning to fly my RV7 over big bodies of salt water. I am priming. If I was in the middle of Australia where its hot, there is bugger all water (not a lot) then I would make the call that the quality of Aluminium Vans uses is pretty good and it should be OK. You may get some spotting but the absence of water means the reation will be localised and won't eat your entire aricraft.

So.....?????
Where are you flying, how humid is it, how salty is it and how clean will the aircraft be on average. ... it "depends" on all of that

To all of that I would add this. How do you feel about not priming? How do you feel about the risk? Do you feel OK about it or do you feel not OK about it. IF your being honest about it, with information, your gut will make the right call.

That is my $0.02 worth.