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  #21  
Old 04-18-2013, 11:18 AM
David Paule David Paule is offline
 
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Some of these primers are just that - primers, with little to no corrosion protection beyond being a barrier film. Because of that, they need to be intact to prevent corrosion.

There are self-etching corrosion-protective primers such as zinc chromate available in the rattle cans. These actually help prevent corrosion.

Dave
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  #22  
Old 04-18-2013, 11:24 AM
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Buggsy2 Buggsy2 is offline
 
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I see two uses of primer.

One, to prepare the metal or fiberglass for the final top coat of paint. This would be typical of visible surfaces. Sealing, corrosion prevention, etc. are not important for this use because the top coat(s) of paint provide protection.

Second, to protect the metal or fiberglass from the environment, wear, and corrosion. This might be used on interior where appearance doesn't matter. An Alclad surface would offer good corrosion protection, though maybe not for physical wear, and all cut edges would not enjoy the Alclad layer.

Seems to me that a primer that's good for use #1 might not work for use #2, and vice-versa. For instance, somewhere I got the notion that the acid-etch primers are great for preparing the surface for top coats, but don't provide protection against corrosion.

Comments?
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  #23  
Old 04-18-2013, 12:22 PM
Aryana Aryana is offline
 
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Location: Western US
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Around my hangar, I've stopped using 7220 and now use the Rustoleum Self-Etching Primer. Feels more durable, and I like the way it goes on...works great IMO, and available everywhere for cheap.

"Etches and primes in one easy step"

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  #24  
Old 05-17-2013, 11:04 AM
MElstien MElstien is offline
 
Join Date: Jan 2013
Location: Kinnelon NJ
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Default Priming using different AKZO products

Hello everyone,

I am about to start a -10 build. My tail came last week and I am finishing the shop. The "engineer" in me still has not made a decision on what primer to use, but I will be priming. I searched the web for information and came upon a few links that actually describe a testing method and results for different priming methods/products. The emphasis was on removing Chromates and making the process healthier and more eco-friendly. AKZO did the study. I wrote to the AKZO person who presented the paper and he said AKZO's recommendation for chromate free aircraft internal priming is to use 2100P003.

Here are the links to the papers. The links look the same but they actually point to two different papers with similar material. I am neither endorsing or recommending anything except educating ourselves on what is available.

http://www.nstcenter.biz/docs/PDFs/M...rown_Adams.pdf

http://www.lockheedmartin.com/conten...rick_Adams.pdf

I will be starting with building some outdoor lamps using Aluminum while I master my sheet metal skills. I will probably order a sample kit of each of the Akzo products and try them, assuming they come in sample kits smaller than gallans.

Regards,

Michael
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  #25  
Old 05-17-2013, 01:59 PM
molson309 molson309 is offline
 
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Location: Longmont, CO
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If allowed to cure long enough the rattle can primers (lacquer based) become very durable and can't be removed easily even with lacquer thinner, acetone or even MEK. I had a patch of the zinc chromate rattle can primer on the fuselage my RV-7A that was a couple years old and when the airplane was painted we could not remove it - we ended up scuffing it and then painting over it. I was amazed it was that tough!

When it is fresh solvents remove it ASAP.
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  #26  
Old 05-17-2013, 04:34 PM
mlwynn mlwynn is offline
 
Join Date: May 2005
Location: San Ramon, CA
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Default Azko vs Steward

My only non-scientific empirical observations are on ASKO nobel (from ACS) and Stewart systems. The Azko is tough as nails. Really hard to scratch, immune to just about every solvent I have tried. The downside is it is fairly toxic. I used a fresh air respirator to apply it. Did the entire internal airframe with it and don't expect to see any corrosion.

I used the Stewart Eko prime under areas that I was painting--also with Stewart Systems. They are really nice people to work with and very helpful.
EKO prime works great under the top coat. It is also fairly tough but can be removed any time with MEK and probably a lot of other solvents. It would probably be okay where you are not going to see any movement but don't rely on it anywhere there might be abrasion or any solvents.

My $02.

Michael Wynn
RV 8 Finishing
San Ramon, CA
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  #27  
Old 05-18-2013, 03:06 AM
SvingenB SvingenB is offline
 
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Quote:
Originally Posted by Buggsy2 View Post
I see two uses of primer.

One, to prepare the metal or fiberglass for the final top coat of paint. This would be typical of visible surfaces. Sealing, corrosion prevention, etc. are not important for this use because the top coat(s) of paint provide protection.

Second, to protect the metal or fiberglass from the environment, wear, and corrosion. This might be used on interior where appearance doesn't matter. An Alclad surface would offer good corrosion protection, though maybe not for physical wear, and all cut edges would not enjoy the Alclad layer.

Seems to me that a primer that's good for use #1 might not work for use #2, and vice-versa. For instance, somewhere I got the notion that the acid-etch primers are great for preparing the surface for top coats, but don't provide protection against corrosion.

Comments?
On the exterior surface filiform corrosion is expected. The top coat, usually polyurethane, is the reason for filiform corrosion. To prevent it a primer is needed.

Traditionally only alodine-primer-top coat would give good protection, but today good primers are formulated without the use of chromates. They give excellent protection against filiform corrosion. The catch is, that is the only thing they are good for (in addition to adhesion of course). It is better to think of primer and top coat as one singe product, used alone they are no good. Following the manufacturers procedures is essential. Typically such a primer needs to be covered by the top coat within hours. Used alone, many primers are porous and will accelerate corrosion because they collect moisture.

Other primers, such as chromated epoxy primers made for aviation applications, are used for internal surfaces. You cannot go wrong with a chromated epoxy primer on the internals. They are strong, durable, and waterproof. When using an "aviation grade" primer, they cover in a very thin layer. The only thing I can think of is it's like shooting small birds with cannons. What Vans are using on the internals on the QBs is probably more adequate for the job, but I don't think it is any easier to apply than an epoxy primer, so why bother?

For the internals, crevice corrosion is the only thing to think about. This corrosion typically occurs between mating surfaces. Chromated epoxy on everything is the brute force method. What is needed is merely a coating between the mating surfaces, but again, it's easier to just cover everything when you're at it, looks much nicer too.

For the internals, it is important that the primer do not create an environment for filiform corrosion. It is important that it is not porous, but repels water. It should be physically and chemically strong. Chromated epoxy is made for this. On the outside, it is much more important that the primer(s) and top coat(s) are applied according to manufacturer's spec.

But then again if you live and fly in a dry climate far from the coast (salt water), store it in a dry hangar, your airplane (or yourself) will become old and useless long before corrosion takes it, even "unprimed". Living and flying near the coast is a totally different matter.

A common misconception about alclad is it doesn't protect at edges and scratches. The corrosion protection mechanism of alclad is galvanic protection. The pure Al or AlZn alloy of the clad is a sacrificial anode to the AlCu alloy of the core. At edges and scratches the clad will corrode, while the core will not. The clad work exactly like sacrificial Zn anodes used in boats.

I am also looking into rattle cans, because it is faster. So far the only solution I have found is one particular that applies in a very thin layer, and then is covered with a thin transparent top coat to repel water and create some physical strength. That is a two step process, and seems a bit silly, but it works. But is it faster than using two component aviation grade epoxy? Another one is a rattle can primer for marine application (aluminium boats and engines, propellers etc). That one has pure aluminium pigments, thus it works just like alclad (galvanic protection). However it is a total mess to apply in anything but a thick heavy layer, so it is useless.
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Last edited by SvingenB : 05-18-2013 at 03:25 AM.
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