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PreKote placebo?

scsmith

Well Known Member
We had to prime a whole bunch of aluminum parts for the stub spars and flap and aileron hinges for our tapered composite RV wings.

I really did not want to use alodine because of the severe environmental issues. (it is hexavalent chromium, which is about like dioxin or plutonium)
The gold standard nowadays is AC-130 Sol Gel, but unfortunately I could only find it in rediculously large quantities that would be enough to treat a B-737, so
...
The alternative we chose was PreKote. They don't tell you much about what it is or how it works, just that it works well. The instructions say to wet the aluminum with PreKote and scrub with a maroon scotchbrite until you raise a grey lather, then apply more PreKote and repeat.

The test for adequate scrubbing is that fresh water should sheet off the part without beading up or even breaking the film. Right. Good luck with that. On the 2024 aluminum parts, we were reasonably successful at obtaining a 'reasonably' break-free film sheeting of fresh water. But on the 7075 aluminum parts, no hope. You could stand and scrub and make grey lather all day long, and fresh water still mostly does not wet the surface on rinse. Multiple repeat cycles, same result.

We primed everything with AKZO (thanks to Dave Howe for splitting a purchase) which of course sticks great. But AKZO pretty much sticks great to anything. So that is no testament for the PreKote.

Beyond the corrosion protection of the AKZO itself, it is not at all clear to me that the PreKote does anything at all. Like I said, they tell you nothing about what it is and what it does.

So, I don't know.....we could probably have just scrubbed everything with acetone and scotchbrite and primed and had the same results. It was a long day of scrubbing grey lather on many many parts, and I just don't know if it was worth the trouble at all.

Next time - AC-130 Sol Gel.
 
Sds

If you look at the sds for prekote its 95% water and 5% Diethylene glycol monobutyl ether. The dgbe is a solvent used in cleaners and other applications. When I started my prep/primer journey I liked prekote until I saw what it actually is, which is nothing special at all. I found you can buy concentrate dgbe online and make your own for a fraction of the cost. I didnt end up using prekote or the home made version. I simply do a light scuff with scotch brite, followed up with a wash in simple green crystal or simple green aviation, then spray akzo. As you have already found akzo sticks to anything. I have scrap parts I tried all different prep methods on and a tape adhesion test shows no difference between dirty no prep at all samples up to meticulously prepped. The test I do is use a razor to cut a grid pattern into the finish, then I use zip system flashing tape which is some pretty sticky stuff. Press it over the scored grid pattern and rip it off. In my case the akzo never failed to adhere no matter how well or crappy the prep was.
 
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But on the 7075 aluminum parts, no hope. You could stand and scrub and make grey lather all day long, and fresh water still mostly does not wet the surface on rinse. Multiple repeat cycles, same result.

Are you sure you removed the transparent protective plastic film? :D

Epoxy primer has excellent adhesion to many substrates, but you do need to remove any oils and water (dessicant before spray gun too). If your ScotchBrite pad gets contaminated, you'll scrub off the oxide layer but can end up smearing a thin film of oil everywhere onto the newly exposed aluminum. The simplest fool-proof prep is to rub with ScotchBrite soaked in Simple Green or other degreaser, rinse with water, allow to dry away from dust (compressed air to get water out of rivets), shoot Akzo within one hour. No need for expensive magic water.
 
"...expensive magic water." Love it.
After doing the acid etch / alodine dance on our whole -9 project, I swore never again. So I ended up doing Prekote on a good chunk of the -8 project. "Self, what the heck am I doing?" A basic scuff and a clean surface with AKZO and papa is happy.
 
Bon Ami

Try scrubbing with Bon Ami. Find it near Comet and Ajax. Very similar except no Clorox.
Scrub with scotch brite till the studs change to a darker color. Very quick. Maybe a minute. Rinse well. Dry and spray within 2 hours.
Cheap. Environmentally safe.
 
Or, if the parts are small enough, put 'em in the dish washer with regular dish washing detergent. Hit start, and go do something useful or fun. Dish washer detergent is one of the best grease cutters out there.

Don't know about a -10, but a lot of the -7's parts, including main ribs, will fit....
 
The simplest fool-proof prep is to rub with ScotchBrite soaked in Simple Green or other degreaser,

Ack, nonono. (green) Simple Green attacks Aluminum; dont use that. Crystal Simple Green (which despite its name is a clear liquid and very different than the green) is an environmentally safe degreaser used for cleaning Al scuba tanks for Oxygen service (where no presence any kind of oil is acceptable) and does not attack it. Thats what I use instead of Prekote (tho I have a full bottle of never used Prekote) for Akzo prep.

All that said, any dishwahsing liquid does the same when used with a scotchbrite for Akzo prep (and I do that on small parts I can clean in the kitchen sink) because as the OP noted, Akzo is very forgiving. All you are really doing is abrading the surface for the Akzo to atch to, and removing hand oils. Dont make it rocket science because unlike other primers, this is not. When you cant see the shine/your reflection on the AL, youre done.
 
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If you look at the sds for prekote its 95% water and 5% Diethylene glycol monobutyl ether. The dgbe is a solvent used in cleaners and other applications. When I started my prep/primer journey I liked prekote until I saw what it actually is, which is nothing special at all. I found you can buy concentrate dgbe online and make your own for a fraction of the cost. I didnt end up using prekote or the home made version. I simply do a light scuff with scotch brite, followed up with a wash in simple green crystal or simple green aviation, then spray akzo. As you have already found akzo sticks to anything. I have scrap parts I tried all different prep methods on and a tape adhesion test shows no difference between dirty no prep at all samples up to meticulously prepped. The test I do is use a razor to cut a grid pattern into the finish, then I use zip system flashing tape which is some pretty sticky stuff. Press it over the scored grid pattern and rip it off. In my case the akzo never failed to adhere no matter how well or crappy the prep was.

I blended my own degreaser using DEGBE, plus a pH adjuster (sodium metasilicate) and surfactant (Dawn). I prepared a number of test coupons that compared the DEGBE preparation vs MEK. All were scrubbed with red scotchbright.

These test coupons and controls have been exposed to a salty, humid seaside atmosphere for 6 months. All of them look to be in good shape. I used Super Koropon, zinc chromate rattle can, and Rustoleum spray paint for testing. All coupons still look good.

I tested the paint bonds by folding additional coupons in half until the aluminum cracked, but the paint/primers remain bonded.

My conclusion? It's the mechanical process of degreasing and scotchbrighting that is the key to success. For degreasing, the relatively safe solution of DEGBE is vastly preferred to handling MEK.

As a secondary test, I used my DEGBE solution to clean an oil drain tray and my grease guns. Incredible results and no smell.

If you look at commercial degreasers, many of them use DEGBE as the secondary solvent (after water) so I am not breaking any new ground here.

V
 
On large flat parts I just use scotchbrite and clean with acetone before priming soon after. On small parks I spray with Alumiprep 33 while scrubbing just in case I miss a nook and cranny. Clean with acetone and prime. Akzo is good stuff but I have seen it peel off of a piece with no prep at all during a test. (there could have been oil I didn't see though)
 
Ack, nonono. (green) Simple Green attacks Aluminum; dont use that.

That's the whole point of paint prep, innit? :eek: It's the phosphates in it that react with aluminum, but the rate is negligible compared to the abrasion of ScotchBrite. If you desire extra adhesion, after rinsing away the abraded and degreased slurry spray the surface with alumiprep (i.e. phosphoric acid, Prep&Etch from Home Depot works the same at half price) and let sit for half an hour to etch under rivet heads too. A good rinse stops the attack and leaves bare aluminum exposed for epoxy to stick to.

All that said, any dishwahsing liquid does the same when used with a scotchbrite for Akzo prep

Bad idea. Most dishwashing mixes leave a film to make your glassware sparkle--that's the "rinse-aid" you add to your dishwasher. You will get a waterbreak-free surface not because it's clean, but because there's low surface tension residue on it. If this is not rinsed off completely, your epoxy will come off in sheets after some years of thermal cycling.
 
Barkeeper's Friend if you can't find Bon Ami, which for some reason is ridiculously scarce around here...
 
I thought about using PreKote, but ended up just scotchbrite and acetone.

Now though, I am considering doing something a bit different for all these dang wing ribs. I am going to dip my ribs in a tray of alumaprep bath, then rinse, then dip them in an Alodine bath, rinse and then prime.

This is way less time consuming than trying to scrub all those complex areas and ridges on the ribs.
 
All good thoughts from everyone on achieving adhesion of the primer, which was never a worry. (although I think the dishwasher detergent is a bad idea).

What I was worried about was getting a good passivation/corrosion protection under the primer. That is what Alodine does, and what Sol Gel does. These are 'chemical conversion coatings' which create a barrier to corrosion.

Why am I worried about this? Because non-alclad 7075 aluminum is vulnerable to "stress-corrosion cracking" and a good passivation coating is important. Fortunately, our 7075 parts are not exposed to the environment much, and are covered in AKZO. So I think they will last a long time.


By the way, another thought. On Simple Green. I have never seen any evidence that regular (green) Simple Green actually attacks aluminum. All I have seen is the aviation (blue) Simple Green says it does not. But that does not really mean the green stuff does. It may be all marketing. I wonder if this is another folk legend like black sharpie pens on aluminum.

Reminds me of the aerosol dried bug remover sold at the auto parts store, that, on the label says that it removes both male and female bugs equally well. Really?
 
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You can find the MSDS for Simple Green here:
https://simplegreen.com/products/all-purpose-cleaner/
I have no dog in that hunt, but it does seem unlikely that Simple Green could do more damage to aluminum than phosphoric acid, as long as it's rinsed off, as is required with the acid. Perhaps the danger is more from it getting between assembled structure, where it can't be completely rinsed away. (Pure speculation...)

On dish detergent: Perhaps the caution is well advised. But when I started building (probably before VAF existed), conventional wisdom was to use Dawn dish soap to degrease parts prior to any anti-corrosion processing. And I can say that washing my small 2024 & 6061 parts in our dishwasher, using my wife's dishwasher detergent (sorry, don't remember the brand) worked great for me. I got perfect etch/alodine on the parts after the dishwasher treatment.

On alodine/painting of interior structure:

A good friend once conducted a test of corrosion resistance of various processes on 2024 test tabs; bare, etch/alodine, various primers, etc. All were wedged in slots in a 2x4, and periodically dipped in a 5 gal bucket of sea water. They were left outdoors about 30 miles from the FL coast, sitting over the open bucket. The alodined sample survived as well as any of the various epoxy, etc coatings. Someone here later published a European scientific study that basically duplicated his observations. My takeaway was that if I alodined interior parts, there was no need to prime them. Saved both money and weight. :) YMMV, etc.

Charlie
 
I thought about using PreKote, but ended up just scotchbrite and acetone.

Now though, I am considering doing something a bit different for all these dang wing ribs. I am going to dip my ribs in a tray of alumaprep bath, then rinse, then dip them in an Alodine bath, rinse and then prime.

This is way less time consuming than trying to scrub all those complex areas and ridges on the ribs.

And how are going to dispose of the used Alodine? NASTY STUFF! PLEASE DO NOT POUR IT DOWN THE DRAIN
 
You can find the MSDS for Simple Green here:
https://simplegreen.com/products/all-purpose-cleaner/
I have no dog in that hunt, but it does seem unlikely that Simple Green could do more damage to aluminum than phosphoric acid, as long as it's rinsed off, as is required with the acid. Perhaps the danger is more from it getting between assembled structure, where it can't be completely rinsed away. (Pure speculation...)

On dish detergent: Perhaps the caution is well advised. But when I started building (probably before VAF existed), conventional wisdom was to use Dawn dish soap to degrease parts prior to any anti-corrosion processing. And I can say that washing my small 2024 & 6061 parts in our dishwasher, using my wife's dishwasher detergent (sorry, don't remember the brand) worked great for me. I got perfect etch/alodine on the parts after the dishwasher treatment.

On alodine/painting of interior structure:

A good friend once conducted a test of corrosion resistance of various processes on 2024 test tabs; bare, etch/alodine, various primers, etc. All were wedged in slots in a 2x4, and periodically dipped in a 5 gal bucket of sea water. They were left outdoors about 30 miles from the FL coast, sitting over the open bucket. The alodined sample survived as well as any of the various epoxy, etc coatings. Someone here later published a European scientific study that basically duplicated his observations. My takeaway was that if I alodined interior parts, there was no need to prime them. Saved both money and weight. :) YMMV, etc.

Charlie

Yes, I think if you etch/alodine after dishwasher detergent, it will take off the surfactant film.

Dawn dish soap is great.

Alodine is very good treatment, no question. It is just a very bad chemical, toxic to the environment, and I think a lot of folks dump the used and left-over stuff down the drain. Please don't do that. Its hexavalent chromium. It is not something that just goes away.
 
And how are going to dispose of the used Alodine? NASTY STUFF! PLEASE DO NOT POUR IT DOWN THE DRAIN

Well considering it has a pretty useful shelf life, if kept out of light, I will probably keep it in my locked chemical cabinet until I need it again.

But, if I need to get rid of it (I have 2 quarts on undiluted solution) I'll take it over to a friends shop. He's an A&P and works with it on the regular. A lunch and some adult beverages and he'll let me add my used Alodine into his disposal tanks.

I've read the MSDS on it, and I do take normal precautions when working with it. I use much of the same PPE as I do when I spray AKZO.
 
I used Super Koropon, zinc chromate rattle can, and Rustoleum spray paint for testing.

V
Ah, Super Koropon. We used it at Northwest Airlines (gone, look it up...) by the gallon. Simple 1:1 mix, ready for spray. Stays put and tough as nails.
 
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