To prime or not to prime

[bmurrish]

After many hours of research and debating, I have decided to prime only non-alclad parts and mating surfaces. I am going to follow the advice of a local EAA Techincal Advisor/RV builder and use Sherwin Williams 988 self-etching primer. I picked up a case at a local auto paint store for $6.70 a can.

Just thought I would throw in my .02 cents to this topic.

[mustang]

The only thing that the self etching primer does is to provide a tooth for a nonpermeable membrane to be applied over it. By itself, the etch does little for anti-corrosion, which is the reason the directions recommend priming. So, if you are going to prime the non-alclad parts with the etch primer, then spray something more substantial over top of the etch. A topcoat would be better than leaving it just etched. An acylic enamel topcoat will provide a true barrier against corrosive salts or acids.

I use the etch primer myself, on large skin surface areas that would be too difficult to etch and alodine before priming. After applying the etch primer, I apply a two part epoxy primer which cures to an all-fluid resistant coating. This primer does not require topcoating on interior surfaces. It forms it's own non-permeable coating over the metal.

The thing about most primer is that it is designed to be topcoated, not to be a stand alone product that acts like a topcoat. The etch primer is usually one of the former, while the epoxy primer is one of the latter. PRIMER, by it's very name, suggests that it is a key for connecting the metal to the topcoat. It "primes" the metal to accept the topcoat which would not stick to the metal very well without help. The epoxy primer by itself sticks very well to the metal if the substrate is cleaned and scuffed properly. Another option is to just do a scrubdown with ALUMIPREP and a scotchbrite pad, rinse well, dry and paint. The optimum method uses the former process and adds an ALODINE bath with yet another thorough rinse to provide the best substrate preparation possible for your primer and topcoat. The etch primer replaces the ALUMIPREP/ALODINE process without providing the anticorrosion properties of the ALODINE. The etch primer provides the tooth for the anticorrosion coating to adhere to.

Cheers, Pete

[captain_john]

I understand it isn't a "sealing primer" meaning the pigmentation isn't as dense as perhaps some others. The "real" job of any true anti-corrosion measure is to place a "durable barrier" between the substrate and the environment.

Let me state that again. The "real" job of any true anti-corrosion measure is to place a "durable barrier" between the substrate and the environment.

Now enter the definition of "durable barrier". Is a coating of wax considered durable? Well, it is for a limited amount of time. A film of motor oil would do the trick too, but that isn't feasible as it collects dust and gets on your clothes. Now paint, that would do the trick! That is where we have all our choices.

Something, is better than noting and applying that "something" in an approved (perhaps in an ASTM standardized fashion) and safe fashion is best.

It really comes down to your exposure to environment, availablility and ease of use. That is why I chose DuPont for now. It may change in the future.

The need for a topcoat for "sealing" purposes is totally optional. One could argue that once the skins are closed in preventing the weather from entering the environment, they may be considered "sealed"... no?

CJ

[Spike]

I tend to agree with you John. Personally Im just going to alodine the small parts, and prime non-alclad pieces and anywhere where parts are mated. All if it will be self etching primer. I know that it wont be as well protected as an epoxy primer, but for me, its good enough. I get reminded of this everytime I seed down the tailcone of my 30 year old Cessna.

[bmurrish]

Correct me if I am wrong, but isn't a self-etching primer a one step process. It etches the surface and primes it as well? Don't make me take back my case of 988. My wife says I am the king of returing.

[captain_john]

Correct me if I am wrong, but isn't a self-etching primer a one step process. It etches the surface and primes it as well? Don't make me take back my case of 988. My wife says I am the king of returing.

It is, and it does. Keep it.

CJ

[David Lynch]

Van's has said that they just Alodine and the Veriprime. And that is all they recommend. That is what I have been doing on the tail. And Van keeps saying "just build the airplane."

Should I be worried now, or should I worry 30 years from now?

Also I have scuffed up the alclad for better adhision of the Veriprime. But I know that this coating is real thin. How much scuffing is too much?

David

[captain_john]

I do the same, except for the alodine.

I use a gray hand pad and just scratch the surface.

CJ

[prestwich]

I've been agonizing over the primer issue for some time now, and I've read most of the archives on this forum and some others. I think there's as much misinformation as there is real information on this topic, just like so many others.

I don't know what's going on chemically between two pieces of metal that are riveted together, or in between the rivet and the hole, and no matter what I do, I know I ain't gonna be able to look in there and see what's going on.

But, I do know this -- aluminum, alclad or not, is self-anodizing. It corrodes almost instantly, but only in a very thin layer, and thereby protects itself from further degradation. So I agree with those who say priming is overrated. Just sitting out there exposed to the air inside the structure, those skins and ribs and spars are going to last longer than I am, and longer than whoever steals my plane once I'm dead.

I'm thinking I'll build the damn thing without any alodining, etching, priming, or painting, and maybe I'll actually be in the air before I need a walker to hobble out to the tarmac. I think fogging in some ACF-50 once in a while is the way to go.

[captain_john]

P-wich,

Where are ya from?

If you live out there in one of them "Square" states, you probably are going to be fine. Out there it is dry and temperate. In Nevada and Oklahoma, the plane would last an eternity without any protection.

Here in New England, things are quite different. The Cherokee Six I fly is a 1969 model. It has spent most of it's life in Sturgis. It came to Massachusetts 4 years ago. There is a row of rivets that is now weeping black corrosion. It makes me wonder what it looks like from the inside. I am not sure what measures Piper took back in the day, but I am guessing they did something? It is in for maintenance right now. Maybe I will look it over while it is in there?

Anyhow, Can I convince you to do a rattle can job, just on the faying edges? It won't slow you down much at all.

I am using DuPont VariPrime ALL OVER, on EVERYTHING and it really doesn't take long at all. I am mixing it and using a gravity feed gun. It is cost effective and durable. It also comes in rattle can as well as does many other fine brands like Sherwin Williams and SEM.

Consider the mating surfaces, if nothing else.

CJ

[cjensen]

p-wich,

I am going to use 988 or NAPA 7220 rattle can on my non-alclad and mating surfaces. I don't think it will take extra time-it dries in minutes! I live in Illinois where corrosion is not an issue. The '68 Cherokee that I had was an Ohio airplane before I brought it to Illinois, and there was NO priming anywhere on that airplane, and it looked like it was brand-spankin'-factory new EVERYWHERE-in the wings, tail, fuselage, and firewall. So, with that in mind, I know the aluminum will be in good shape many years down the road with nothing, but with the little amount of time it takes to rattle can some self-etching primer on, it should last a lifetime.

.02

[prestwich]

I live in Santa Barbara, which is on the coast and subject to fog. But given the cost of hangars here, if I ever actually get my plane built, I may end up keeping it inland where it's dryer.

I spent some time on the phone the other day with the ACF-50 guy - I think his name is Mike - and he said that although he recommends "following the manufacturer's instructions" with regard to priming, he also said that primer and paint and alodining are brittle and will crack with any sort of flexing. Airplanes are going to flex some in flight, no doubt about it. He didn't mention cracking with the pounding of a rivet gun, but I wonder whether that might be enough to expose bare aluminum to the elements long before the plane is finished.

The ACF-50 system seems pretty credible, to me, and it makes more sense than painting in some ways. He wasn't advocating it as stand-alone corrosion protection, but I'm still considering going that route.

Have any of you considered sending your ready-to-assemble parts out for commercial anodizing? That also seems like a relatively inexpensive and easy way to offer some protection. But since I haven't seen it mentioned anywhere, maybe there's drawbacks about which I'm unaware.

[captain_john]

The way I see it, the only drawbacks to anodizing everything... is cost and the utter impracticality of it.

Oh yeah, it'd look wicked cool!

In order to make it feasible, you would need the whole anodizing setup in your back yard. A paint shop is easier to pull off.

CJ