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Primer Testing (Akzo)

DanNiendorff

Well Known Member
Hi All,

Ealy in my 14A build here and looking for ways to improve my process. After looking over all the options, I've decided to prime as follows:

1) Subtructure Parts -
a) Alumiprep Dip then rinse
b) Alodine Dip then rinse
c) Akzo prime
d) dimple

2) Skins (Internal Side only)
a) Scuff
b) Acetone wipe until clean
c) Akzo prime
d) dimple

I needed to answer several questions to make my process the most efficient...

Question #1:
Akzo recommends a single wet coat to a thickness of 1.0-1.3 mils. As a novice painter - what does that look like?

Question #2:
Akzo has an induction time of 30 minutes - recommended (not required). Does the induction time improve the final product for my purposes? Or should I just go ahead and shoot it.

Question #3:
The Akzo tech datasheet says it is 'dry through' at 4 hours, and does not list a cure time. It does list a topcoat window of 1-48 hours. Which makes me think it is fully cured at 48 hours. It looks like common practice is to wait about 12 hours prior to dimpling. What is the minimum time to dimple?

I did an experiment to answer these questions. I set up two groups of aluminum coupons, which were all treated alumiprep dip/rinse then alodine dip/rinse and air dry. Once dry each coupon was marked with an "x" in sharpie. I set out each group of coupons in a line and sprayed a very light (airbrush) coat along the entire line. Then repeated the light coat along the entire line, excluding the first coupon. Then the second, third, etc..... So that the first coupon had a single light coat, and the second had two, the third had three, etc..... I did this with the first group of coupons without the 30 min induction period, and the second group with paint that had undergone the 30 min induction time.

I measured the mil thickness of each coupon (using Carsys meter), in order to establish a visual reference for myself while spraying to achieve the proper thickness.

I also sprayed a strip of aluminum (same prep steps) with each group, and dimpled it at intervals after spraying to see at what point dimpling could be done, and if the 30 min induction changed the interval necessary.

Here are the results:

2hqJbaF


I think my experiment answered my questions as follows:

#1 - Thickness visual measure achieved. Hopefully, the pic can help someone else with this and save them repeating the experiment.

#2 - It does look like the induction time lead a more consistent spray thickness over the spread of the coupon group (although It could also have been my technique). It also looked like the final dimple kept better coverage in the induction group. I scratch tested the coupons that fell in the proper thickness range for the induction and non-induction group and could not tell any difference. So it looks like 'recommended' improves some things in the final dimple. And I would use it if I were planning to dimple after priming, however.....

#3 - I found that dimpling after priming will lead to splitting of the primer even after the full cure time. It decreases over time. Here it is early after priming...

2hqGmu9


And then after 24 hours of cure......

2hqGmt2


So from this, I conclude that if dimpling after priming it would be wise to use the 30 min induction, and wait at least 24 hours (or more) prior to dimpling if the akzo will be the only protection. However I'm considering using an Alodine 871 Touch-Prep treatment to the hole after dimpling to ensure that areas of primer distortion are protected from corrosion. I imagine this is not a surprise to anyone with experience in this. But I thought posting my data points might help someone without the experience, such as myself.
 
Don't see your pictures, but:
You'll probably want to scuff a bit with the Alumiprep stage.
You should not be able to see through the Akzo. This will give you the proper dry film thickness (I also did some coupons and tested them for thickness with a meter)
I usually dimple after Alodine, but before Akzo. Just because I don't tear up the abrasive pads so quickly during the Alumiprep treatment/scuff! Dimples really tear up the pads.
I would not put on Akzo without an etch or scuff. The skins aren't that bad to Alumiprep, as you can wipe it on and scuff it without worrying about catching the rinsewater. Alodining skins is a bit of a pain.
Now if you're going to polish the skins, then don't put any Alumiprep on them, inside or outside, because it will get on the outside.
The induction time gives the epoxy time to "kick" before you shoot it.
Akzo dries really fast. You can handle the parts almost immediately. Just don't stack them, etc. because you will get marks.
 
The dimple tests look to be too thick. What is the mil thickness on those? Especially the second one. It looks like you have solvent pop/crazing.

Your color swatches match what my experience has been. It should go to just opaque and smooth. If it goes glossy or wet it?s too thick. I get right at 1 mil dry thickness when I measure with mics.

Scuffing when etching makes a huge difference for adhesion.
 
My .02... Alodine AND Akzo is repetitive, you are putting corrosion protection (Akzo) over a layer of corrosion protection (Alodine). Either / or is fine and doing both is doing more work for little or no benefit. I scuffed then dimpled then acetone wiped then applied Akzo for all parts and didn't differentiate structure from skins as far as prep or application.
 
I've dimpled after priming almost all my parts so far in the build. I have not experienced the flaking, or cracking of primer around the dimples as shown in your pictures . What are you using to make the dimples..C-Frame, or DRDT-2? I use the DRDT-2 or hand squeezer.
My usual process is to wipe down the shiny, deburred parts with MEK, or equivalent (unfortunately, MEK by far the best), to remove red lettering and any oils that may have accumulated during prep. Then the parts are scuffed with Pre-Kote and maroon scotch brite pad. Parts are dried, usually with a blow dryer, to reduce water spots. Priming is done the same day. The Pre-Kote specs say you need to prime within 24 hrs, but I don't trust it that much.
I have no problem with the 30 minute induction time. I don't move that fast anyway and usually use that time to finish setting up.
I couldn't view your smaller pics that well, but for the ones showing the dimples the primer seemed to have an unusual appearance/texture.
As far as the performance of the Pre-Kote... I'm not sure. Some believe it is nothing more than an expensive cleaner of some type. I have done a few pieces without the pre-kote (just cleaning, scuffing and wiping with MEK) and they didn't seem to "hold" the primer as well. Could have been in my head. The specs for the Pre-Kote says it " provides a polar/non-polar molecule that attaches itself to permanently imbedded contamination; this molecule attracts the coating". Again, I'm no expert, but I've just always used it with good results. That said, it is still very time consuming, and a pain the butt.
Recently, for prepping some of the fuselage parts for priming, I experimented with the acid etch, followed by alodine dip. No doubt, this is probably the best, and a little easier, primer prep process. However, because it can be dangerous to your health, and disposal issues, it's not the magic bullet either. Set-up and clean-up can be time consuming until you develop a "process". I currently have three buckets of contaminated liquid sitting outside in a safe location waiting to evaporate down to an amount where I can mix with concrete and dispose.
Sure you need to prime?, not sure I will on my next build:)
 
My .02... Alodine AND Akzo is repetitive, you are putting corrosion protection (Akzo) over a layer of corrosion protection (Alodine). Either / or is fine and doing both is doing more work for little or no benefit. I scuffed then dimpled then acetone wiped then applied Akzo for all parts and didn't differentiate structure from skins as far as prep or application.
I disagree.
I did some coupon testing years back with both a salt fog chamber, and hanging outside in coastal FL for a year. Alodine-only is okay if you can keep dust, etc off of it, but you really need an organic coating to give a layer of protection against dust etc. The dust particles hold moisture against the surface and allow corrosion cells to take hold. My alodine-only VS skins had pitting on them from dust after being stored up near the garage ceiling for a couple years. Just on the dusty top surface.
All of the alodine-only test coupons had corrosion starting on the edges after being in the weather. The etch/alodine/epoxy primer coupons were dirty, but pristine after a year+. The bare alclad coupons were a mess.
For us salt-coast folks, I recommend some sort of organic barrier coating. Akzo, Epibond, and similar two part epoxies are pretty bulletproof.
 
Just my opinion here after doing a complete airplane build primed with Akzo-Nobel.

  • Definitely wait the 30 minutes induction time before spraying and stir it a lot during that time. Mix more than you think you need and don't put it all in the spray gun -- keep some in reserve. You will invariably find some areas that just need a bit more after it starts drying.
  • I originally did the alodine step with the tail kit, but abandoned that with the wing kit. As others have noted, it is overkill to do both. Alodine is nasty stuff and the less you expose yourself to it, the better.
  • Dimpling after priming works fine, but you really have to wait a day (24+ hours) to let the primer fully set up.
  • Scuff using a 3M scotchbrite pad with alumiprep, and then rinse, rinse, rinse and make sure it is completely dry before priming. You can also wipe down the aluminum with acetone to get it cleaner before/after scuffing.
 
Thanks everyone for the input.

I measured the mils on the dimple coupons, and the last one was indeed very thick. Nearly 3 mils - so that explains the appearance. The first one was not overly thick, but was dimpled early after priming, which explains it appearance I believe.

I should note that none of the akzo splitting was visible to the naked eye. I could only see it when magnified. So I will continue to watch for that in my QC.

I dimpled the 1 mil coupon tody, which is now >48 hrs after priming. It looks good to me.....

48854615681_867f163587_b.jpg


For me the lesson is to do the 30 min induction and wait for a full cure prior to dimpling. Or if the timing works better, dimple prior to priming. All of the male die side of the dimples looked fine after just a short time, it was only the female die side that had issues, so I do think setting a rivet could occur fairly shortly after priming a pre-dimpled part. Compression does not seem to be so much an issue as expansion.
 
Update

I mentioned above that I typically dimple after priming and have not experienced any flaking or cracking of the primer.....well that has changed. I recently primed a fairly large batch of fuselage parts and, because of the large amount of time and effort involved in my normal prep process, I decided to give the alumiprep and alodine method a try. My goal was to reduce the prep time, particularly in the mechanical scuffing stage. This stage seems to take the most time and effort, especially on parts with numerous nooks and crannies. I thought I could save some time by chemically etching with the alumiprep. Additionally,the alodine process, which I have avoided in the past to due its hazardous qualities, I thought would give me a little extra time between the prep and the priming stages. Using my old method I felt I needed to prime the parts the same day of prepping avoid oxidation buildup. This made for some very long prime days.
So, to prep for this alumiprep/alodine method; I bought plastic tubs that would accommodate most of the parts for the fuselage, new alodine and alumiprep, and several gallons of distilled water.
My process:
1. Wipe all parts with MEK to remove contamination.
2. Completely submerge the part in the alumiprep for approx.. 3 mins, agitating as required. (I checked for water break surface the first few parts)
3. Double rinsed parts in tap water by submerging in buckets and containers.
4. Immediately submerged parts in alodine bath until light golden color. (approx.. 3-3.5 mins)
5. Double rinsed in separate containers using distilled water.
6. Hung dry for 24 hours
7. Primed parts with AKZO

Following this process, the parts looked great and I was happy with the results. I felt the prep/cleanup was a little more involved but overall time was saved by reducing the scuffing stage. Plus, I didn’t feel rushed into priming the same day because the aluminum would not oxidize.
However, a week so after priming, I began to dimple the parts and noticed the primer was chipping and flaking around the dimpled areas. I had not experienced this issue in the past when using my previous prep method. As an additional note, there were a few longer parts that I was not able to fit in my alumiprep/alodine tubs. I prepped these parts using my old technique of scotchbrite and prekote scuff. These parts were primed the same day using the same batch of primer. These parts were later dimpled the same day as the alumiprep/alodined parts. However, there was absolutely no flaking or chipping of the primer on these parts.
My unsubstantiated thought is that the chemical etch does not provide the same amount of “tooth” for the primer to grab. Possibly adding a scotcbrite scuff to the alumiprep stage would correct that. However, the whole purpose for me to use the alumiprep was to skip the scuffing stage.

I would be interested to hear if others that use the alumiprep/alodine prep method , and dimple after priming, are getting the same results of chipping/flaking primer around the dimples. Unless, I can determine the cause of the primer flaking, I will return to my previous prep procedure using the prekote and scotchbrite scuff.

DSCN7167.jpg
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The two parts (long and short pieces) at the end of the work bench are the parts shown in the following photos. The long piece was prepped using scotchbrite/prekote, and the smaller piece alumiprep/alodine.

DSCN7143.jpg
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The pics above and below show the short piece that was prepped with alumiprep and alodine. The primer did not hold during dimpling.
DSCN7141.jpg
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DSCN7162.jpg
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The pics above and below show the longer part that was prepped with scotchbrite/prekote. Primer in perfect condition after dimpling.
DSCN7163.jpg
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Induction is for starting the chemical reaction necessary for curing. Without induction, you run the risk that some resin molecules don't start the reaction once sprayed out in a thin coat and then don't fully cure. I suggest you follow it, especially if you end up putting down thin coats. Once a reaction starts, it won't stop unless the temp goes below critical. Be cautious of this, as some epoxies can go dormant (i.e. stop their curing process and not restart) if they drop below the critical temp during the earlier parts of the curing process. I use SPI and it has a 7 day open window and the risk of dormancy is the first 24 hours. Epoxy is still a decent primer if it doesn't fully cure, just not as strong or corrossion resistant, hence the reason it is a recommendation.

The open windows is the time in which the epoxy will still chemically react (i.e. chemically bond) with other similar, reactive components. It is not the final cure time.

no alodine here. It is not necessary with an epoxy primer, assuming proper prep is followed. I also don't use alumiprep, though it is not a bad idea. I simply scuff the AL parts with a maroon pad and soapy water solution. Scuff untill 100% water break free surface. Blown air dry, then a final wipe with wax and grease reomver and let dry for 30-60 minutes. Then get epoxy on within a couple of hours. This is critical and something often overlooked here. I sometimes clean the parts with laquer thinner first when getting rid of oils or the blue plastic adhesive residue.

This is for external painting. For interior, I just generally clean and scuff, followed by a self-etch primer. However, I am in the camp that this step is really belt and suspenders, so don't care if I had an adhesion issue somewhere down the line.

Larry
 
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I disagree.
I did some coupon testing years back with both a salt fog chamber, and hanging outside in coastal FL for a year. Alodine-only is okay if you can keep dust, etc off of it, but you really need an organic coating to give a layer of protection against dust etc. The dust particles hold moisture against the surface and allow corrosion cells to take hold. My alodine-only VS skins had pitting on them from dust after being stored up near the garage ceiling for a couple years. Just on the dusty top surface.
All of the alodine-only test coupons had corrosion starting on the edges after being in the weather. The etch/alodine/epoxy primer coupons were dirty, but pristine after a year+. The bare alclad coupons were a mess.
For us salt-coast folks, I recommend some sort of organic barrier coating. Akzo, Epibond, and similar two part epoxies are pretty bulletproof.

I agree that alodine only is not a great solution, it is designed to be top coated for protection, though the top coat doesn't require anti-corrossion properties. However, had you compared a sample piece with standard prep and epoxy primer, I think you would have found it to hold up just as well as the piece with alodine and epoxy.

Larry
 
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