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PreKote - Preparation of alclad surfaces. 'Official' feedback from the manufacturer

O

Oliver

Well Known Member
After reading differing opinions on whether the alclad layer should / needs to be removed, when PreKote is to be used for surface preparation, I sent an e-mail to Pantheon Chemical, the manufacturer of PreKote.

Nate, a process engineer at Pantheon, patiently answered my questions and also allowed me to post his statements here in the forum. I thought this might be beneficial for other builders, as removing the alclad layer with a Scotch-Brite pad is a very tedious job, what would made the use of PreKote rather unattractive.

This is what he told me, quotations are italic:
- All his statements refer to clad aluminum (not the lacquer with the same name). The shiny layer of pure aluminum, on top of the aluminum alloy, Van's aircraft are made of.
- The PreKote process involves the use of a ScotchBrite for removal of oxides and agitation of PreKote.
- Fortunately with PreKote, you do not need to deviate from the application procedure to remove this oxide layer.
- Removing the oxides present on the Alclad layer is important but removing it completely is not necessary.

I think this makes the application just as easy as the preparation for rattle can primer or the SW P60G2, I initially wanted to use. Instead, I will now give PreKote + AKZO 463 a try.

Besides of the supposedly fast and easy application, I like what I found on their website:
- PreKote is extremely safe and is a non-hazardous, non-toxic, and quickly biodegradable technology with a pH of approximately 10–11.5. Chromated conversion coatings (such as Alodine) are highly corrosive acids (pH of 1.3–3.0), contain a known carcinogen and do not biodegrade.
- After passing numerous paint system specification tests by the US Air Force, Aerospace OEMs, and USAF flight-testing, the USAF specified PreKote as the only replacement for chromated conversion coatings in the T.O. 1-1-8, the umbrella Technical Order for aircraft painting.
- PreKote has proven to be compatible with any epoxy or urethane primer or topcoat from major manufacturers.

Nate referred me to this training video: https://youtu.be/K1oHQwY1GYE
He also sent me an application guide, sadly this forum does however not allow attachments. The guide at Aircraftspruce is very similar, the main steps are identical: https://www.aircraftspruce.com/catalog/pdf/09-00825instruct.pdf

Oliver
 
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What the ??

OK. I just can't resist.


Who on earth thinks that the alclad layer needs to be removed?? Huh???
 
Oliver, thank you for the information. Prekote is on my list of future things to try.
 
Oliver,

The USAF reports (that I could find) only approved the pre kote with certain primers and topcoats that have a specific chemistry for a "system" result.

Did the PreKote Rep talk about a listing of recommended primer/topcoat combinations? (or specific constituents?)
 
Just a FYI. When I attended a PPG Concept class, PPG chemist approved the use of PreKote with their primer.
 
I have been using it for a year or so and it's a great time-saver compared to acids and Alodine, not to mention less toxic. I had wondered if it was in use by the USAF or just approved by the USAF, till I noticed a placard on that beautiful B-52 at OSH this year that said it was prepped with PreKote. Two passes with Scotchbrite/PreKote and you are done.
 
arffguy said:
[...] Who on earth thinks that the alclad layer needs to be removed?? Huh???
Click to expand...

I was wondering the same. A gentlemen, who stated in another thread that it is a pain to remove the entire alclad layer with a Scotch-Brite, however cited another user who made this statement:
"The decision to remove the Alclad or leave it is a can of worms. If you have an aircraft painted at a paint shop they will sand and etch the skins to make them water break free, which removes the Alclad layer. The reason they do this is because almost nothing will stick to Alclad and your pretty paint would peel off if not removed. All of my wing ribs were thoroughly hand scuffed with maroon scotch brite prior to applying the Akzo primer which removed most but not all of the Alclad. Most of my prep work on the ribs would not pass a water break test. I did several paint adhesion tests on the ribs with very good results, which shows that the Akzo primer has excellent adhesion on parts as long as the surface is slightly abraded and clean. Since I will be painting over the Akzo on the exterior surfaces I really need to have good primer adhesion and sanding with 220 or even 320 make me sleep better at night."
Click to expand...
Source: http://www.vansairforce.com/community/showthread.php?p=911387#post911387 (#23)

Digging deeper into this topic, I found other, similar statements. If this would have proven to be true, I would have Alodined the parts instead, as I wouldn't have felt comfortable about removing a protective layer and as the effort / benefit ratio wouldn't have been in favor of PreKote.


BillL said:
[...] The USAF reports (that I could find) only approved the pre kote with certain primers and topcoats that have a specific chemistry for a "system" result.

Did the PreKote Rep talk about a listing of recommended primer/topcoat combinations? (or specific constituents?)
Click to expand...

I did not ask, because they are very specific on their website, saying that "PreKote has proven to be compatible with any epoxy or urethane primer or topcoat from major manufacturers".


We are just about to start working in the training kits. As I want to prepare them just us the actual RV-14A kit, it was important to me to do as much research in advance as possible. In any case, the training kits and the plane will be spayed with AKZO. I will start with PreKote on the training kits and see how it goes. If I am unhappy with the results, what I do not expect, I could still fall back to Alodine for the actual built.

I also ordered a 3M Accuspray system. With this, cleaning the spray gun seems to be a matter of seconds. Not exactly cheap, though...
https://youtu.be/PrH_6ttt06k
 
Prekote is great stuff. But if youre near me, I'll sell you my un used un opened gallon. I bought it, but don't use it cause I O2 clean scuba tanks and use the same stuff I use for that work.
 
noelf said:
Just a FYI. When I attended a PPG Concept class, PPG chemist approved the use of PreKote with their primer.
Click to expand...

That is good to know, but do you remember which primer that still meets the bazillion hour salt spray scratch test on copper bearing aluminum alloys? Any "aerospace" aluminum alloy (except 2024) works with a wide range of primers. 2024 (copper alloy) seems to be very particular. PPG has have more formulations (primers) than a cosmetics counter.

http://www.asetsdefense.org/documents/Workshops/ASETS2012/4/Spicer - For Web.pdf

This is required reading if one expects to prevent corrosion.
 
BillL said:
[...]
http://www.asetsdefense.org/documents/Workshops/ASETS2012/4/Spicer - For Web.pdf

This is required reading if one expects to prevent corrosion.
Click to expand...

Very interesting. This is what I took away from reading through the presentation:
- Old fashioned Alodine (MIL-DTL-81706) seems to be less picky regarding the primer and the top coat than PreKote or other chrome free surface preparation products.
- The chrome free Alodine 5200 provided only insufficient protection.
- If combined with the right primer and top coat, PreKote offers a similar performance to Alodine (MIL-DTL-81706).
- PreKote achieved the best performance with the AKZO chrome free primer Aerodur 2100 MgRP (see slide 23).

I found on Lockheed Martin's website another presentation in which the combination of PreKote and AE 2100 was tested. The results were similar:
http://www.lockheedmartin.com/conte...obel-Chromate_Free_Coatings-Patrick_Adams.pdf

AKZO also specifically states PreKote in the datasheet of AE 2100 as a possible product for surface preparation: https://www.silmid.com/MetaFiles/Silmid/97/97d69237-3e43-4f78-9075-8e205c073561.pdf .

Now, the question is, what these results mean for the combination PreKote / AKZO 463-12-8 vs. Alodine (MIL-DTL-81706) / AKZO 463-12-8? Another option might be to use AE 2100, what I however couldn't find in any online shop...
 
I've been using PreKote on Alclad and bare alloy as surface preparation for AkzoNobel 10P4-3NF epoxy primer with EC-117 curing solution.

Surface preparation was in accordance with the PreKote web site instructions, and primer application was with a LVLP gun following the Akzo data sheet instructions. So far all has worked very well.

I apply primer before dimpling and riveting, and both those operations have left the cured primer undamaged.
 
Well done Oliver!

Oliver said:
Very interesting. <snip>.
Click to expand...

Yes, thanks for the extra work. New/updated and commercial information I could not find in the last research session. Wish I had seen this before purchase of the last zinc chromate primer purchase, but at least, I have a proven system, even if ancient.

This is an excellent and solid foundation for new builders to protect their planes with a proven product, and with DATA! I bet a few calls to Akzo will track down a military depot for purchase. One might have to set up an account (I did w/PPG). They use phones not internet!
 
I sent an email to Pantheon a few weeks ago to ask questions about prepping parts for PreKote. I was specifically asking about the possibility of using Alumiprep prior to applying Prekote (mostly on smaller parts that I found are difficult to prep with scotchbrite pads). I realize that I may have been too concerned about removing the alclad layer during the PreKote process and I may change my thinking on that. I also asked about ST-1, a new product I saw on their website.
This is the reply I got from Nate at Pantheon.
"ST-1 does not have an particular advantage over PreKote. ST-1 was designed as sister product to PreKote to enter other markets besides aerospace. If you are looking for other Pantheon products to make your painting life a little easier, I suggest the PreKote Wipes. They are effective for touch ups and small painting applications as they don?t require a ScotchBrite or rinse water.

Regarding the small parts process, I think it will work well as the Alumiprep will etch the surface and provide an area for PreKote to grab onto. As long as you get a ?water break free? surface, you know you?ve applied PreKote correctly."

Some thoughts I had on his reply:
I am not familiar with how the PreKote wipes work (are they slightly abrasive), but think I will try and acquire some for a test.
I know some think it's crazy, but I may experiment with the alumiprep prior to PreKote on the small, hard to scotchbrite parts.
I think it all comes down to his last statement " As long as you get a ?water break free? surface, you know you?ve applied PreKote correctly." Now..how much prep/scrubbing it takes to get this "water break free surface", is still the question of the day and debatable. I think it may be a situation of Good, Better, Best. I may end up trying for the "Better" category:).
 
Oliver said:
For the immersion application of PreKote, this company actually uses some kind of 'acid etching fluid' instead of Scotch-Brite: http://www.lcaaaviation.com/pantheon-chemical_application.htm

I would be interested to learn about the results of your test with Alumiprep and PreKote. :)
Click to expand...

Hey Oliver, Sounds like you and I are all over this PreKote prep process..that's ok, takes all kinds of people to build an airplane in their garage:). We may have to "scale back" a bit if we want to actually get one in the air some day:eek:.
I'll let you know how the alumiprep works out. I will probably also prepare a few parts with just a slight scrub with scotch brite to "rough up" the alclad before the PreKote process. I'll see if that will also provide a water break free surface.
 
Mudfly said:
[...]We may have to "scale back" a bit if we want to actually get one in the air some day:eek:. [...] I'll see if that will also provide a water break free surface.
Click to expand...

Well, I am actually thinking that some initial research might save us a lot of time, without any sacrifices regarding the result. I also don't want to spend a considerable amount of time and money on prepping and priming, only to see the primer start to peel of after a few years, because I chose the wrong products or because of insufficient preparation of the parts... :eek: ;)
 
Process

Mudfly said:
Hey Oliver, Sounds like you and I are all over this PreKote prep process..that's ok, takes all kinds of people to build an airplane in their garage:). We may have to "scale back" a bit if we want to actually get one in the air some day:eek:.
I'll let you know how the alumiprep works out. I will probably also prepare a few parts with just a slight scrub with scotch brite to "rough up" the alclad before the PreKote process. I'll see if that will also provide a water break free surface.
Click to expand...

Here's another easy and cheap prep to a water break free surface.
Scrub with scotchbrite and BonAmi cleanser. Rinse and dry.
 
wirejock said:
Here's another easy and cheap prep to a water break free surface.
Scrub with scotchbrite and BonAmi cleanser. Rinse and dry.
Click to expand...

+1
Abrasion and removal of oxidation layer are key to creating a strong bond with Epoxy Primer. You really don't need the expensive chemicals. I am a big believer in mechanical adhesion. I used Bon Ami with a maroon pad and followed with a thorough sanding with 80 grit on a DA for my exterior (2 hour max delay before spraying). Epoxy grip was very strong.

Larry
 
lr172 said:
+1
Abrasion and removal of oxidation layer are key to creating a strong bond with Epoxy Primer. You really don't need the expensive chemicals. I am a big believer in mechanical adhesion. I used Bon Ami with a maroon pad and followed with a thorough sanding with 80 grit on a DA for my exterior (2 hour max delay before spraying). Epoxy grip was very strong.

Larry
Click to expand...

Aannnd... we're back to alclad removal.

:)
 
rv7charlie said:
Aannnd... we're back to alclad removal.

:)
Click to expand...

I don't think this is what lr172 said / meant. Just like an aluminum alloy, the layer of pure aluminum (alclad) only oxidizes on its surface. Sanding it with 80 grid paper, would however indeed quickly go through the alclad layer.

Frankly, though, 80 grid sounds a bit brutal and not really necessary to me, considering that the bonding of the primer with the surface happens on a molecular level. I also understand that products like alodyne or PreKote, with their water-like viscosity, ensure that the entire surface, down to the tiniest little pocket, gets treated and protected. They also provide other functions, like to prevent corrosion from undermining the primer. Just look at the US Air Force presentation above - the differences between the various pre-treatments are pretty shocking.

It appears as whether the process and the required efforts are exactly the same, if I use BonAmi or PreKote. No offense, but if I put $140k into a plane, I don't want to take the risk that the primer peels off after a few years or gets undermined by corrosion, only to safe a few hundred bucks. YMMV, of course.... ;)
 
80 grit on a power sander=alclad gone.

I thought that the whole reason for alclad on 2024 is that 2024 is (relatively) highly susceptible to corrosion and the pure aluminum alclad provides a protective layer, in that after the surface oxidizes, it is relatively immune to further corrosion (compared to the 2024, at least). I also thought that a conversion coating *on the alclad* followed by paint would effectively give 3 levels of protection. Grinding off the alclad seems like a pointless process at best, and much more likely, a destructive process, because I'd now be totally dependent on the painting process.

But I'm just a redneck who's only painted parts....

Charlie
 
Alclad

appeared to be important on the longevity of parts on a Corsair I took apart in a salt marsh. Any panel that was intact when it hit the ground was still intact 30 years later. Any panel that was scratched through the Alclad was... gone.

Any "tooth" required for attachment of primer can be attained with a scotchbite pad.

My opinion: 80 grit paper is used on composites, not Aluminum.
 
I never saw these follow on posts, so thought I would reply with some facts. The clad layer on .025 material is .0012 (5%) and is not microscopic. You'd be surprised how much effort is required to remove .001" of metal with sandpaper. Further, I used the 80 grit on a DA, which is comparable to 180+ by hand or with an orbital sander. Further, I did tests sanding MUCH more aggressively than my final work and removed less than .0005" of material in those tests.

https://www.alcoa.com/mill_products/catalog/pdf/alloy2024techsheet.pdf

In cases where I may have penetrated the clad layer, I have no concerns as my epoxy primer is no less corrosion resistant then the clad layer and any scratch likely to get through all the layers of poly and epoxy is also likely to go through the clad layer as well, netting the same result.

Larry
 
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