Zinc Chromate Wash

**This is NOT meant to start a primer war. However I do want to hear everyone's opinion and hope we can disagree cordially **

If you don't want to read the whole post, my questions are at the end.

I'd like to start by saying that I'm a firm believer in being an independent thinker and coming to your own decisions after you've done your research. However that doesn't mean you can't ask people for their opinion, it's actually a form of research.

I asked some builders at the airport for some priming suggestions. The idea I liked best was to use a Zinc Chromate Wash so I researched it further and found the following:

Zinc chromate is simply a corrosion resistant pigment that is added to certain coatings. In the case of a wash primer, the phosphoric acid actually reacts with the metal, whereas in the case of zinc chromate this is not the case. In dry, low humidity weather the zinc chromate does not do anything other than remain in the coating. On the other hand, when the humidity in the air increases, or on a rainy day moisture for the air penetrates the primer coating and slightly dissolves the zinc chromate. The dissolved zinc chromate solution now does react with the underlying metal surface and forms a passive layer (like a blanket|) that prevents corrosion. As soon as the weather becomes dry again the zinc chromate no longer pays a role ... at least until the next time it rains, when the process is repeated. If there are frequent cycles of high and low humidly the zinc chromate will eventually be depleted and soon after you might start to see corrosion of the metal..

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http://www.paintcenter.org/rj/jul02k.cfm

Needless to say I'm a lot less enthusiastic about using it since I live in Florida. The airport I will be based at is in proximity of the ocean, and I plan on using a tie-down so corrosion protection has my attention.

I like the idea of using the wash because it's easy to apply. Would using a topcoat of epoxy such as AKZO on it solve the depletion issue? Of course the article doesn't state how long that would take so we could be talking decades.
The reason I'm looking at the AKZO is that from what I've read it seems to be the toughest thing around, simple to use and white which makes it look nice. The cons are that it takes time, space, and has got to weight a ton to prime everything correctly. I'm still unsure as how I would use it, just some area or everything.

My first train of though was to only prime what Van's tell you to prime because the alcald will protect the rest. This is simple and keeps the airplane light which I really like so I haven't completely disregarded that option. However, I want to do the "right" thing and I'm asking others for their opinions so that I can a better decision.


What did you use, why and what is your local climate? (If you didn't build, how much did you factor in primer and the location of the plane.)
What are your thoughts on a Zinc Chromate wash + AKZO and how would you use the AKZO?
What are your thoughts on just prime what Van's tell you to?
What would you use in a humid climate next to the ocean if you were parked outside?


Thanks all for reading this long post and sharing your opinion. I'm sure this will help me make a better decision and will have me armed and ready when I call the manufacturers for more information.

Paul (Not sleeping while waiting for an RV-7 emp kit and trying to make up my mind )

What did you use, why and what is your local climate? (If you didn't build, how much did you factor in primer and the location of the plane.)
What are your thoughts on a Zinc Chromate wash + AKZO and how would you use the AKZO?
What are your thoughts on just prime what Van's tell you to?
What would you use in a humid climate next to the ocean if you were parked outside?

I used Sherwin Williams vinyl wash primer (zinc chromate version). I live on the Pacific coast near the ocean which is quite wet in the winter.
I would not use anything over the wash primer. My discussions with the SW factory indicated that the primer 'locks up' and becomes a moisture barrier without a top coat. It is easy to apply, light, and does not need the surface to be scuffed, which saves a ton of preparation. The acid will etch into smooth alclad.

Also, it does not change the dimensions of the parts much, so things fit better than with a thich layer of primer.

I would use it again because it is so easy to use and effective. Because of the simple prep and self-etching properies, it is much more likely to adhere in the difficult spots where corrosion is more likely.

BTW, a friend who is an Air Canada mechanic says that the best anti-corrosion treatment of all is to spray the interior with ACF-50 every two years. It penetrates and stops any corrosion and penetrates the seams where moisture accumulates.

Vern Little

Paul,
You certainly asked a question that will get plenty of opinons, here's mine.

I live on the west coast of Florida, as in; I can see the waves, and when it blows from the west I get salt spray on the car. In this climate you cannot build fast enough to leave it bare and go the ACF50 route later. Trust me on this. Maybe in Arizona, but not in humid Florida.

Dust is your enemy. Why? Because after you finish that shiny empenage and hang it up in the rafters, when you take it down in two years every speck of dust and dirt will have acted as a moisture sponge and started a nice little corrosion cell. The surface you could see on the bottom will probably look fine, but wait till you run your hand over the side that got dusty.

I'm going to ramble a bit now.
Back in the old days when I was building my RV-4 and the only thing pre-punched was the builders manual, I had more time than money. I knew it would take me at least seven years to build it and I didn't want the first pieces finished to be corroded scrap by that time. I also happend to have access to a aerospace environmental lab. So, I made some test coupons.
1. Bare metal (alclad)
2. Etched and Alodined (chromic acid conversion)
3. Etched, Alodined, and primed with Randolph Epibond two-part epoxy primer

All were subjected to 200 hours salt fog and then hung in my backyard for a year or so. Yep.
The bare alclad did not do well.
The alodined did very well in the salt fog but did not hold up in the weather. The edges really took a hit.
The alodined and epoxy primed was basically untouched. Tough stuff.

Unfortunately I didn't have Variprime, so I don't know how that would do. My experience with it now leads me to believe that it is very good, but probably not as good as the etch/alodine/epoxy.

My original conclusion was that for enclosed areas the etch/alodine would be fine, so I primed selectively; mating surfaces and ribs, spars etc. Unfortunately I didn't take into account dirt and dust. The alodined skins on my empenage got pitted after being stored for a couple years.
BTW, all the alodining was done via immersion.

MY New conclusion: Anything that will keep the dust off the bare aluminum will help significantly. Etch/alodine with an epoxy topcoat is bulletproof. The epoxy can be very thin, see-through. Is it worth all the effort? It is very time consuming and a PITA (not to mention toxic), but weigh that against picking up a vigin piece of you plane and finding corrosion has already taken hold.

The other thing to consider is that any exterior priming may be temporary. If you have a pro-shop paint your plane, it is likely they will strip off the primer. Hmmm, wonder if latex house paint would suffice?

What am I doing on my current project? Variprime on interior parts. Might do etch/alodine/epoxy or Variprime on the exterior skins. Still debating. I still do etch/scotchbrite on the parts I Variprime. I just don't trust any paint to stick to shiny aluminum.

Now if I lived in a dry climate, I would prime only what was required and do the ACF thing after completion. Although I bet repainting a plane that's been ACF'd is daunting. I hear that stuff weeps out of every nook and cranny.

Paul Thomas said:

My first train of though was to only prime what Van's tell you to prime because the alcald will protect the rest. This is simple and keeps the airplane light which I really like so I haven't completely disregarded that option. However, I want to do the "right" thing and I'm asking others for their opinions so that I can a better decision.
)

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Paul,
I'm glad you brought this up. I'm building mine in humid Georgia, but I have a dehumidifier in the shop. The aircraft will be hangared so I feel that corrosion will be a "controllable" issue for me. I'm leaving the Alclad alone and only priming what Van's says.
My plan is ACF-50 and/or Boeshield T-9. I've seen the ACF-50 work on Civil Air Patrol C-182s in my Kauai Squadron (when I'm on island). On Kauai we mist the insides every 100 hour inspection. The ocean and surf are pretty close, but corrosion is kept at bay. The big issue we have is corrosion under the exterior paint. Little blisters on the trailing edges that soon become big blisters.
Apparently ACF-50 is very similar to WD-40 with a few key additives for eating corrosion. I like WD-40, so I think I'm going to love ACF-50.
John

Paul,
I second Mike Weed's opinions. I live in SE Florida. We have several RV-6s locally, which are always hangered. Even with constant hanger storage, internal corrosion starts after about 4 years here, without Alodine. These aircraft are based at HWO, which is about 5-6 miles from the ocean.
It always amazes me at the perverse logic of forgoing the use of Alodine and Mil Spec epoxy primers based on "it adds to much weight", with the rationale that they can apply ACF50 (or similar), later. This stuff weighes a LOT more than epoxy primers do. In addition, dirt and debris become imbedded in it, which adds even more weight. Having to re-coat every few years will leave you with a ship which will weigh much more than a properly Alodined and primed ship.
The real rationale for forgoing Alodining and priming is to save LABOR. Unfortunately for those of us who live in Florida (or New Zealand) this is not a practical option. Testimonials from owners on the US west coast mean nothing. They have the salt air, but not the heat we have here in Florida (or New Zealand).
New Zealand builders who purchased quick build RVs also report that relying only on the SW wash primer applied by Vans, got no more than 4 years (hangered) before serious corrosion was found.
Local builder Peter Laurance had to re-skin his rudder because he had not Alodined it under the primer. This occurred while he was building the wings to a 6A. The rudder was stored in his attic in Miami Beach, at the time.
The reality is that you live in the corrosion capital of the world. Anything less than the best (Alodine and Mil Spec epoxy primer) will mean more work, in the long run. The Germans have a saying. "The lazy man works the hardest of all".
Charlie Kuss

White AKZO?

The reason I'm looking at the AKZO is that from what I've read it seems to be the toughest thing around, simple to use and white which makes it look nice.

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Does AKZO really come in white? If so, where can I get some?

BTW, my experience with it is that it's hard to get off with sand paper!

tomcostanza said:

Does AKZO really come in white? If so, where can I get some?

BTW, my experience with it is that it's hard to get off with sand paper!

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It doesn't. I read to much about too many products and ended up confused.

This is getting to be frustrating and it's a process I really overlooked because I thought "priming, that easy, just spray some stuff and you're done". I called several vendors but I wasn't able to talk to the people I need to because they're still on their way back from OSH. I'm sure many of you are doing the same and I hope you'll chime in because I'd like more info, especially from FL people.

Vern,
Do you have a phone # for S&W. I've been calling their 1-800 # and keep getting an answering machine.

John,
I don't want to deal with the hassle to spray ACF-50 at every conditional. Not only do you have to buy the product every year, but you have to find someone to borrow the sprayer from or buy one. Add to this some airports don't want you to spray that stuff because it make a big mess.

Mike,
Just to make sure I understand clearly, your opinion is that the inside and outside should be alodine and primed during construction. When it comes time to paint the aircraft, strip the primer and prepare to paint as you normally would?
Have you given any thought to painting the components as you finish them? This way you wouldn't have to prep the outside twice.

Charlie,
When you say corrosion, is it correct to assume we're speaking about the beginning of the process when we can see the brown is starting to appear. From there how long does it take to get a part that one shouldn't fly with?

You both make a very good argument to prime everything which is something I hoped I wouldn't have to do.

Everyone comments are apreciated, thank you very much.

Paul, sorry that I don't have the phone number for S-W handy. Below are my notes from 2002.

Ensure that you are trying to contact Sherwin Automotive, not Sherwin-Williams consumer products.

Vern


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Primer I chose Sherwin Automotive E2G 973 Vinyl Etch Wash Primer for the following reasons:

* It is available locally (Vancouver B.C.) and has good web support for data sheets.
* It is a corrosion inhibiter (zinc chromate), not just a moisture barrier.
* Due to the inclusion of some epoxy polymers, the primed surface ?locks up? to provide a moisture barrier. Not as good as true epoxies, but much easier to use and lighter weight.(See note)
* It is light weight (no fillers)
* It does not absorb moisture Some primers, such as the Sherwin E2G980/983 have talc fillers to allow sanding. The talc will absorb moisture.(See note)
* It does not require sanding or roughing up of the aluminum substrate, just cleaning.(See note)
* It has good technical support from the 1-800 number

Note: information obtained from Sherwin-Williams.

Your primer choice may be different, but I based my decision on readily available information from the Sherwin Automotive website and technical support staff. I have found very little useful information available online from PPG, DuPont and others. link: http://www.sherwin-automotive.com/products/. Just to be fair the Sherwin-Williams consumer web site is one of the worst on the internet (not just for paint!). The automotive web site is a bit better.

I will admit that the E2G 973 does not provide a nice finish like a pigmented primer.

Thanks Vern! That was quick. I'll give them a call and see. I do like what I see by reading your notes.

Anyone else interested in the phone # it's: 1-800-798-5872.

Edit: Just got off the phone with one of their representative. She was obviously reading from a script that they are giving them and not willing to say anything but "we don't recomend E2G 973, you need to use DP800 or E2A933" and she couldn't answer most of my question which she told me I'd find the answer to once I read the info she's sending me.
According to her, the products she recommended with a top ooat are all I would need.

When I first asked about the zinc chromate products, she told me they didn't sell them anymore because they're dangerous to your health, but E2G 973 is still for sale.

She also advised me to talk to their aviation department who have a JetGlo solution which I've found online.

Paul, I actually talked to a product engineer, who called me. Not just a phone response person reading from a script.

Also, I talked about materials (steel, aluminum) before telling him it was an airplane. I think once they hear 'airplane', they steer you to JetGlo products. the vinyl etch primer is used a lot in welding shops where they will weld structures, shoot the primer then store them outside in the weather for several months.

I think they are phasing out zinc chromate primers because they don't want a visit from Erin Brockovitch (personally, I wouldn't mind...).

BTW my exterior paint is JetGlo over primer/alodine/etch.

Anyway, contact me in about 10 years, I'll let you know how effective it's been . I do have some parts in my hangar that I primed three years ago and have been exposed to dirt, dust and moisture... still in perfect condition.

Good Luck, Vern

Paul,
I am not a professional painter, so keep that in mind.

A pro paint shop has to guarantee their work and they have no idea how well the primer was put on, etc. Therefore they play it safe and take it down to the bare metal. At least that?s what happened to a friend who had Variprimed his exterior skins.

If YOU are going to do the final painting, then there would be no need to strip the primer off. You know you did a good job, right? Just clean everything and scuff the primer to get some ?tooth? to it.

Most advocate waiting until the plane is complete and tested before final paint. Again, a tradeoff. Lay on your back to paint, or paint in pieces and risk damage or mismatch.

It is MY opinion that etch/alodine/epoxy primer is the most effective system. It is also a pain to do, time consuming, toxic, and slows down your progress. I usually wait until just before riveting the parts together to do the priming. It?s all a tradeoff.

Of course, the real question is: How long does the plane need to last? Most of us don?t have fifty years of flying left. Shoot, most of us don?t have ten (especially by the time we get the plane done ).

fl-mike said:

Paul,

[...]

Of course, the real question is: How long does the plane need to last? Most of us don?t have fifty years of flying left. Shoot, most of us don?t have ten (especially by the time we get the plane done ).

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If that's all I've got left, I'm in real trouble or taking way too long to build this thing since I'm only 21.

I went to Barnes&Nobles tonight to see if they had any books on aluminum corrosion and how best to prevent it but I struck out. Hopefully I'll have better luck at the library tomorrow.