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  #1  
Old 08-04-2015, 05:03 PM
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s10sakota s10sakota is offline
 
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Location: Westland, MI
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Default Can't find any 'real' information...

Ok, I hate to ask this, but....

I can't seem to find a real scientific answer. Does priming airframe parts with NAPA 7220 offer corrosion protection or can water (or moisture) soak through the cured primer?

There are 1,000 opinions on the net about this. Does anyone here have any scientific data to answer this question? Are there any chemists or scientist who can weigh in on this? I've done a lot of searching for this info and one guy will say "Oh yes definitely" and the next guy in the next post says "must be topcoated..." But no post I ever read said, "I am a chemical engineer who studies paints and primers and these are the facts..."

If it doesn't offer any corrosion protection, then I just wasted a lot of time and money priming the inside of the wings, tail, and fuselage with 7220
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Last edited by s10sakota : 08-04-2015 at 05:06 PM.
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  #2  
Old 08-04-2015, 08:56 PM
BillL BillL is offline
 
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Quote:
Originally Posted by s10sakota View Post
Ok, I hate to ask this, but....

<snip>

If it doesn't offer any corrosion protection, then I just wasted a lot of time and money priming the inside of the wings, tail, and fuselage with 7220
Why are you asking? If it is poor, but better than nothing, then what are you gonna do?

Why not just put corrosion inspection on your annual and take notes. After all, many, airplanes last several decades with no primers at all.

If you want something better for the future, then look for something with chromates. Zinc chromate is hard to get and in large enough quantities and at a high enough price to dissuade mere morals from purchase. Strontium chromate is the latest formulation and is available in Corlar. Sold in gallons at about, well, maybe $180/gal + an equal cost for a gallon of harder. It is an epoxy formulation. I got old epoxy primer with zinc chromate and it is about gone. Alodine is a chromate conversion process and a good corrosion inhibitor. Easy to use, but comes with it's own hazards. Good base for primers and top coats too.

Go to googleS for scholarly articles for Navy documents on corrosion prevention of copper/aluminum alloys (2024). Good luck, finding useful factual data is hard to find.
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  #3  
Old 08-04-2015, 09:22 PM
David Paule David Paule is offline
 
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Check the MSDS to see if any corrosion-inhibiting materials are used. If not, then it's just a moisture barrier, better than nothing, especially if both surfaces are covered.

The standard corrosion test specs, if I understand it right, are kind of an accelerated testing. If it passes them, then the protection is quite good. Different people will have different opinions for what's required, and those will often be based on where and how the plane is stored, and what they've seen. Some climates are relatively benign and some are relatively severe, when it comes for corrosion.

If you're looking for good protection, it's worth going to the specs, especially the coating specifications, and studying them.

Dave
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  #4  
Old 08-05-2015, 06:41 AM
David-aviator David-aviator is offline
 
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How long do you plan to live?
Nothing lasts forever, build the airplane and go fly it. Your body will wear out before it will with or without priming.
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  #5  
Old 08-05-2015, 10:00 AM
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FresnoR FresnoR is offline
 
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Build a couple bowls made from the same aluminum.

Use primer on one of them.

Fill both with saltwater, keep full for a month or two.

See what happens...

Take pictures
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  #6  
Old 08-05-2015, 11:31 AM
David-aviator David-aviator is offline
 
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Quote:
Originally Posted by FresnoR View Post
Build a couple bowls made from the same aluminum.

Use primer on one of them.

Fill both with saltwater, keep full for a month or two.

See what happens...

Take pictures
Good test if building a submarine
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  #7  
Old 08-05-2015, 12:05 PM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by David-aviator View Post
Good test if building a submarine
...or living in Florida...
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  #8  
Old 08-05-2015, 12:41 PM
Boomer506 Boomer506 is offline
 
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Location: Stanwood, WA
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Default DuPont VeriPrime 615S

I don't want to start primer wars either, but this is my second airplane and the sponge brush application of VeriPrime on interior surfaces before dimpling and assembly just really seems to pass the easy/cheap/effective tests for me.

A single quart mixed thin will do all the interior surfaces of my RV-8. I mix a thimble full if I need to by drawing out of the can with a tongue depressor and sponge brush on thin. It dries in 15 minutes or less and I can dimple or rivet away almost immediately.

It is self-etching, so if I have any contaminants on the surface, it cuts through them and still adheres. It has chromate in it, so it serves as the sacrificial annode so-to-speak.

It does not seal, you need a top coat to do that. But for interior surfaces, I think it is the bees knees. I have attached a picture of how my skins look when treated this way. Again, the key is to mix thin. So you will need two quarts of activator to go with your one coat of primer. As you can see, I do my exterior surfaces also. It seems to help prevent scratching during the build.

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  #9  
Old 08-05-2015, 02:48 PM
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Ironflight Ironflight is offline
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Years back, someone here on VAF coated a number of aluminum coupons with various primers and hung them out in the weather for a long-term test. It might have been Dr. Horton (sounds like something he'd do, carefully checking each one every day for tell-tale signs of corrosion...). He might have exposed one to Spotted Cow, just for effect.

So there IS real-world test evidence, but I don't know how to search for it. Might be way, way back in the Primer forum.
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  #10  
Old 08-05-2015, 05:17 PM
PCHunt PCHunt is offline
 
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Quote:
Originally Posted by Boomer506 View Post
I don't want to start primer wars either, but this is my second airplane and the sponge brush application of VeriPrime on interior surfaces before dimpling and assembly just really seems to pass the easy/cheap/effective tests for me.
I Googled veriprime because I wasn't familiar with it, and got lots of hits on cows and food. Better results were obtained with: Variprime.
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