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Green zinc chromate primer as final finish

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Peer Ebbighausen

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As the subject line says, I'd like to leave the green primer exposed in some areas on my badland ultralight build, if for no other reason than I like the look of it.

It wouldn't be heavily exposed - I'm thinking the engine mount and a few engine brackets. Is there any harm in doing so, or does this have a very short "shelf life" if not sealed with a topcoat?
 

Turd Ferguson

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If you paint steel, it will eventually rust. Zinc chromate is also toxic and carcinogenic. Lots of restrictions on it's use these days and the alternative is zinc phosphate, which is of dubious equality.

My favorite primer is the old stits EP-420 but not sure what the availability is for that anymore.
 

Kyle Boatright

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I'm no doctor or toxicologist, but Zinc Chromate use does have its risks.

That said, with proper precautions (spray it outside, stand upwind, wear a mask, wear long sleeves) and the small quantities you're talking about, I'd use it. In fact, I do use it in small quantities.

Knowing what I know, I wouldn't be comfortable doing it the way the factories did in the 40's with an open thousand gallon vat of the stuff and a worker using a garden hose to spray it on a fuselage weldment, or another worker dipping hundreds of parts into it all day long.

Everything has some element of risk. The only question is your threshold for acceptance.
 

N804RV

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I painted the interior of my RV-8 with SEM gray primer. Used a light tack coat followed by 2 wet coats of SEM. Then, I covered it with 2 light coats of Eastwood 2k matte clear. All rattle can. People keep telling me I'll have trouble with it. But, I have a test piece that's been tossed around the shop and hung outside in the rain for a couple of years. And, it still looks pretty good.
 

wsimpso1

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MANY of us use green epoxy primer on steel and stop there, with excellent results.

Worker hazard? It goes on a lacquer and it is two-part epoxy. Gotta wear gloves and a solvent mask to apply it, then ventilate the solvent away. Once that is done, it dries for handling quickly. The epoxy takes a week to fully cure, so gloves and mask are a good idea for a few days if you are sanding. After that, everything in the epoxy is pretty well encapsulated in the epoxy. Epoxy is common in medical implants and tools, once cured, it is sturdy in the body and quite inert.
 

Dan Thomas

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Primers are hard to keep clean. They're a really flat finish, for bonding of the topcoat, so smudges from grubby hands will mark them up. Some primers aren't really weather-resistant, either. They rely on the protection of the topcoat.

But I do like the look of the green or yellow zinc chromate. We can still get the stuff here in Canada. Endura has it as an epoxy, which can make me retch if I get one breath of it. And I'm allergic to a lot of epoxies, too, from using some of the earlier epoxy glues in the '70s with my bare hands, against all recommendations. Screwed up my immune system somehow and I began reacting to some other things, like dairy products. And I do love cheese and ice cream so much and have to take them in tiny doses.
 

Dan Thomas

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Thousands of seaplanes are coated inside with zinc chromate.
Cessna primed internally all the airplanes that had the extra float fittings in them. They had stainless control cables, too.

When they went back into production in 1996 they primed the inside and outside of every airplane, and every airplane got stainless cables, which last about a fourth as long as galvanized cables.

I've spent hours inside them fixing plenty of stuff. I often found the chromate flaking off in the pre-restart ariplanes. It points to a lack of proper metal prep before priming. That aluminum needs to be real clean, needs acid-etching and alodining and the prime needs to go on real soon after that, before the oxygen starts working on it again. Aluminum really doesn't want anything sticking to it. The earlier stuff appeared to be a laquer of some sort, while the restart airplanes had epoxy. They primed the prepared skins and other components before they riveted everything together. Prevents corrosion between the joints.
 

BBerson

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Yeah, scrub with phosphoric acid etch and rinse to remove the mill oil and get a proper bond.
 
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Peer Ebbighausen

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Thanks all.

I opted for the chromate because I was told by Better Aircraft Fabric that it's one of the better bases for their adhesive. (Zinc phosphate apparently is a poor bonding agent to the point where they will no longer honor any warranty's on their product if it's used.)

I used the Aircraft Spruce rattle can stuff. Not cheap, as far as rattle can paints go!
 

Victor Bravo

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The rattle can one-part zinc chromate will get eaten or loosened by a solvent based fabric glue. Little scabs of green ZC primer will come away from the bare steel with the glue as you remove the fabric to do a repair or re-cover. Or the solvent will make it dissolve.

I am sorry to be the bearer of bad news, but that ZC primer will not last long... if you like the look of primer, then it needs to be the 2 part Stits / Poly-Fiber epoxy primer.

Sorry...
 
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malte

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I have used the epoxy based 2k MIPA system on my Lake. The zinc phosphate primer Mipa 100-20 is available in all colours. If you want to have it "raw" outside, i'd use a thin PU top coat. I use Mipa 250-90. You can have that in the same colour as the primer if you want, but its much easier to clean than the primer.

If you want to use Oratex, you can use EP/UP systems. The weak link can be identified by the glue test described in the manual. The adhesive is not solvent based.
 

Heliano

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Zinc Chromate is banned here. We use strontium chromate or two-component epoxy primer instead. The epoxy primer works well with for aluminum/galvanized surfaces but can also be used on steel.
 

mquinn

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I am a fan of epoxy (and can tinted to look like zinc chromate - even with a non sheen) - but the KEY is prep - proper surface prep is CLEAN of all debris AND have proper surface - too smooth is actually a bad thing as the epoxy must make a mechanical connection.
 

Peer Ebbighausen

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Kyle, I have noticed that...once it "sets up", it seems quite durable, but it takes some time.

VB, thankfully the Oratex adhesive is not solvent-based. The manual actually recommends it.

I guess I'll paint the engine mount to avoid any future complications. Thanks all!
 

Doran Jaffas

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As the subject line says, I'd like to leave the green primer exposed in some areas on my badland ultralight build, if for no other reason than I like the look of it.

It wouldn't be heavily exposed - I'm thinking the engine mount and a few engine brackets. Is there any harm in doing so, or does this have a very short "shelf life" if not sealed with a topcoat?
Coming from experience leaving some of it exposed you'll be surprised at how long it does last. Also coming from experience, make sure you have a positive pressure air mask on when you're spraying it. When I sprayed an airplane long time ago with it I didn't even think about it until after I got out of the spray booth and it could have turned out really bad.
 

Dan Thomas

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The rattle can one-part zinc chromate will get eaten or loosened by a solvent based fabric glue. Little scabs of green ZC primer will come away form the bare steel with the glue as you remove the fabric to do a repair or re-cover. Or the solvenbt will make it dissolve.

I am sorry to be the bearer of bad news, but that ZC primer will not last long... if you like the look of primer, then it needs to be the 2 part Stits / Poly-Fiber epoxy primer.

Sorry...
On certified airplanes, the Poly-Fiber STC demands that their primer be used under the cement, as too many others will debond under it. That should serve as a warning to homebuilders.
 

robertbrown

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Zinc chromate primer provides great corrosion protection because of the chromates. The chromates are depleted by exposure to moisture so a topcoat over the primer will provide the best protection by minimizing chromate depletion. How fast will the chromates be depleted? The USAF paid for a study of the zinc chromate on 30+ years old C-141's. Most samples tested still had a higher chromate level than the new "reduced chromate" epoxy primers that are now being used. The only places where the chromates were depleted to the point of ineffectiveness were where standing water had collected. That's what drain holes are for.
 
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