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

s10sakota said:

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

Click to expand...

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.

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

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.

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

FresnoR said:

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

Click to expand...

Good test if building a submarine

David-aviator said:

Good test if building a submarine

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...or living in Florida...

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.

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.

Boomer506 said:

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.

Click to expand...

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.

Building for "life".

David-aviator said:

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.

Click to expand...

I think this quote highlights a significant problem in the Experimental category. When builders' talk about building "for life", what they are often referring to is the builder's flying life, not the aircraft's flying life.

There are plenty of old builders out there who will be struggling to fly for another 10 years....but the aircraft they have built may go on for decades.

If builders keep cutting corners on their priming there are going to be literally thousands of rust bucket RVs out there eventually.

There are some builders who say: "I don't need to prime 'cause I'm living in the desert in Arizona". I say: "Yeh, but when you eventually sell it, it will probably go to some-one who lives in Florida".

There are also some builders who say: "Cessna and Piper didn't prime them for decades and some of them are still OK". I say: "Yeh, but most of them are now suffering from severe corrosion problems that are threatening structural integrity and costing GA hundreds of millions a year in restoration costs and as a result both Cessna and Piper have now learnt their lesson and now use an epoxy primer on all of their singles."

From time to time you read posts on VansAirforce about RV builders who are getting corrosion on their Alclad parts even before the aircraft is finished.

When you take on the building of an Experimental aircraft the product you produce reflects, for good or for bad, on the Experimental category until that aircraft is written off the register. The aircraft becomes your legacy to the Experimental category.

Boomer506 said:

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.

Click to expand...

If one was thinking about going out and purchasing primers today, be sure to check the msds first. The older VariPrime contained zinc chromate which was known to be very effective against corrosion for aluminum containing copper (2024). Many good primers of a few years ago have had this material removed. 615S included.

Look to the marine industry which still has primers with this readily available. Aviation is available, but in larger containers. Also, priming and not top coating can lead to different primer solutions. Some very effective ZnCh primers are not 2K (urethane or epoxy) formulations and will lift with many modern (as in current production) 2K topcoats