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-   -   PreKote placebo? (https://vansairforce.net/community/showthread.php?t=159780)

scsmith 04-21-2018 08:19 PM

PreKote placebo?
 
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.

Mlidzct 04-21-2018 08:50 PM

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.

Aluminum 04-21-2018 09:19 PM

Quote:

Originally Posted by scsmith (Post 1254656)
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.

scard 04-22-2018 06:51 AM

"...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.

wirejock 04-22-2018 07:37 AM

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.

rv7charlie 04-22-2018 07:49 AM

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....

maniago 04-22-2018 07:53 AM

Quote:

Originally Posted by Aluminum (Post 1254663)
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.

vlittle 04-22-2018 08:03 AM

Quote:

Originally Posted by Mlidzct (Post 1254660)
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

jcarne 04-22-2018 08:20 AM

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)

Aluminum 04-22-2018 09:10 AM

Quote:

Originally Posted by maniago (Post 1254736)
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.

Quote:

Originally Posted by maniago (Post 1254736)
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.


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