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Paint/priming part prep

wirejock

wirejock

Well Known Member
Greg Baron posted this link in another thread. I encourage anyone painting or priming to watch. Most is about the powder coat process but the portions on cleaning are particularly fascinating. Thanks Greg.
http://youtu.be/x7onZGqrYyY


I am starting the painting of my interior and want the paint to stick so I performed some unscientific tests based on the video. I'm no scientist or engineer so take it for what it's worth.

I used several samples. Primed samples with SWP60G2, bare aluminum scuffed with maroon scotchbrite, bare untouched aluminum. I have quite a scrap pile.:eek:

First test was to dribble water with no treatment. All samples beaded up and ran off.
Second test was parts wiped with solvent. Lacquer thinner, Acetone, MEK. Acetone worked best but water still beaded up with all three.
Third test was parts cleaned with powdered cleaner, Ajax, per the video. Results were quite surprising. After drying, all parts wet thoroughly and water formed a film on the surface. Even the bare aluminum with no scuffing at all wet well. Same for the P60G2 sample.
Other parts were tested with Dawn and the product I've been using, AeroGreen 4110. Dawn was ok. 4110 was almost as good as Ajax but only on scuffed parts. Neither did much for bare aluminum.
I suspect the best process is a combination scuff/cleaning process with a good cleaner followed by a thorough rinse. Many of you use this method already. One bare sample cleaned with gray scotchbrite and Ajax was as good as the pre-scuffed part cleaned with Ajax so one step could be removed by combining the scuff and clean process.
The other issue is time. The best paint adherence will be within 2 hours of cleaning. I will test the parts tomorrow to see how time effects the drip test.
More tests will be done including paint tests but it looks like the cheapest product on the shelf worked the best. Ajax is a whopping $1.
 
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I have painted most of my exterior. Process is to scrub clean with scotch pad and Ajax type product. Rinse thoroughly. Immediately prior to paint I sand with 80 grit. If I can't get to paint within 2 hours of sanding, I scuff with with maroon pad. Final clean with wax & grease remover, tack then spray epoxy primer.

Aluminum begins to oxidize 1-2 hours after sanding/scuffing. It is the oxidizing that causes the bulk of adhesion problems. Alodine will keep it from oxidizing, but I didn't want to use acids in the garage.

The 80 grit sanding helps to provide a stronger mechanical bond for the epoxy. Two coats of epoxy primer completely fills the scratches.

Larry
 
80 grit?? Better really pray the epoxy is perfect, cause the alclad is sure not going to be there to help. :)

For degreasing small parts (even wing ribs), a dishwasher is your friend. Load it up, use regular dishwasher detergent, and run a cycle. Works much better than even the mythical Dawn, and you can build (or fly, or watch TV) while it works its magic.

Still need to scuff it with scotchbrite, of course.

Charlie
 
Aluminum oxide

The enemy in adhesion is the aluminum oxide layer. Removing it requires some sort of scuffing and cleaning. Powdered household cleaners have an abrasive. It also reforms within a few hours of removal so timing is necessary.
I'll run some parts through the dish washer to see how it performs. Unfortunately, most of what need paint won't fit. Testing is fun. So is drinking beer while testing.:D
 
Have you tried . .

phosphoric acid prep? The same stuff used before alodine coating.

I dip my parts for about 3 minutes and rinse with distilled water to avoid dissolved solids, then blow dry. I used it decades ago for prep before primer for an aluminum hood on an MGB. It remains a recommended prep for painting.
 
Acid

BillL said:
phosphoric acid prep? The same stuff used before alodine coating.

I dip my parts for about 3 minutes and rinse with distilled water to avoid dissolved solids, then blow dry. I used it decades ago for prep before primer for an aluminum hood on an MGB. It remains a recommended prep for painting.
Click to expand...

Yes. Phosphoric acid removes the aluminum oxide layer.
 
rv7charlie said:
80 grit?? Better really pray the epoxy is perfect, cause the alclad is sure not going to be there to help. :)

For degreasing small parts (even wing ribs), a dishwasher is your friend. Load it up, use regular dishwasher detergent, and run a cycle. Works much better than even the mythical Dawn, and you can build (or fly, or watch TV) while it works its magic.

Still need to scuff it with scotchbrite, of course.

Charlie
Click to expand...

It's 80 grit on a DA. It's similar to 180 by hand. I did a series of tests and it removes well under .001" and I feel the Alclad is still there. Not that it matters, as after priming the primer is the protective layer, not that alclad. If Alclad was a better corrosion inhibitor than Epoxy primer, no one would prime anything.

If you are concerned about parts that don't have alclad, what did you use for non-sheet aluminum in your build? Most is not alclad.

Larry
 
Prep follow up

I checked the parts cleaned yesterday. The bare aluminum still wets reasonably well. I would be comfortable priming it. However, adhesion will be best within the 2 hour window.
I also tested the process used on everything to date. Scuff with scotch brite followed by a cleaning with AeroGreen 4110. It wets the same as the scotchbrite/ Ajax prepped part. Nice to know.
Finally I prepped a part with a very light scuff with gray Scotchbrite and washed with AeroGreen. It also wet well but deeper scuffing would probably yield a stronger mechanical bond.
My unscientific conclusion is the scuffing needs to be deep enough for a mechanical bond but need not be so severe it erodes all the Alclad or requires multiple coats to fill.
The two processes are independent. Scuffing provides the mechanical bond. Cleaning removes the aluminum oxide layer and oils.
The aluminum oxide layer is very thin. Ajax removes it easily with no further scuffing needed. You could combine the two processes and get both a good mechanical bond and removal of the aluminum oxide layer but paint needs to be applied within 2 hours of the operation. A big stack of wing ribs would probably exceed the time limit.
I suspect scuffing with most of the recommended aluminum prep solutions would be equally successful. It's pretty hard to beat the price and performance of Ajax.
I plan to adjust my prep process such that the cleaning is done day of painting. Scuffing can be done any time.
I've seen and read this Ajax process in several places researching aluminum pain prep processes. Please run your own tests. I am no chemist, professional painter or engineer.
YMMV
 
Cleaning parts, Jet Flex adherence test results

Follow up on the adherence test.
Sherwin Williams Jet Flex Solvent Based
Sample one primed with Sherwin Williams P60G2
Sample two bare aluminum.
Both samples scuffed with maroon scotchbrite
Both samples cleaned using a gray scotchbrite pad and Ajax.

Parts are cured. They were still tacky but a day in the furnace room at 90 degrees finished the cure.
Sweetie worked on both samples for a while with her deadly sharp fingernails and couldn't make a scratch in the paint or get paint to lift at the taped edge.

A sharp key cut into the paint down to the metal on both samples as expected. Primer or no primer made no difference. Just as durable as a car factory paint job.

I know by experience. Some turd keyed my 2001 Honda S2000 a few years after new. All the way down to the metal. It had a three stage factory paint job. Primer, metal flake base and clear. I left it so no future turds would key it.

A flat blade razor was the last test. Paint could not be peeled up from either sample. It scraped off but not like the surface tension was lost. I could not get a razor blade under the paint to peel it off.

I'm satisfied with bare aluminum scuffed with maroon scotchbrite then cleaned with gray scotchbrite and Ajax. Note: There is no corrosion protection when sprayed directly on bare aluminum so any parts subject to weather should be primed with a product designed for aluminum protection and compliant with a top coat.

Next is HVLP spray tests. After one post to VAF, several calls to the store and reading tips on the internet, I've got a pretty good starting point.

More results once spray tests are done.

sb7ek4.jpg
 
Jet Flex Solvent Based HVLP test

Follow up HVLP spray test.
Keep in mind, I'm not a professional painter. YMMV

After a few attempts, the magic sweet spot is achieved (or as close as possible). Results are for my gun and will vary with gun and tip size.
Ambient temp: 70 degrees
Humidity not measured

Jet Flex Solvent Based
7 parts paint : 1 part catalyst : 8 parts reducer
100% reduction

Sherwin Williams recommends 4 parts max reducer or 50%. The SW rate was too thick to spray smooth but at 100% the surface came out very smooth. If you want some texture, shoot at 50%.
The paint and catalyst are really thick. Paint must be shaken for at least 15 minutes prior to painting to mix the pigment and flattening agent thoroughly. After measuring, mix the spray pot very well before spraying.

Wipe parts with a tack cloth immediately prior to spraying.

Paint was dry to handle within minutes.

HVLP DeVilbiss Finishline FLG3
35 psi at the inlet
3/4 turn out on paint valve
1 turn out on fan control

Spray a very thin light coat to start then add additional light coats till full coverage is achieved.
Do not leave it in the gun any longer than necessary. It sets up fast and is a mess to clean up. I always use syringes to measure paints to reduce the mess. They are labeled and clean easily with lacquer thinner or acetone.
 
I hope this isn't a too silly question, but it is driving me nuts as I just can't decide whether to prime, using Sherwin William P60G2 or if I should first Alodine the parts and then spray AKZO epoxy primer.

The ease of application is the main decisive factor. Frankly, though, when I see with how much effort some people prepare their parts before they spray P60G2 or rattle can primer, I don't see how this saves much time over alodining and then spraying AKZO. In this case, I would rather use AKZO primer.


wirejock said:
Yes. Phosphoric acid removes the aluminum oxide layer.
Click to expand...

Don't products which contain phosphoric acid, like Alumiprep 33, also take care of the etching of the aluminum, so that it does not also have to be mechanically scuffed with a Scotch Brite pad? I also understand that it degreases the surface and that it is therefore not necessary to also clean the part with acetone and silicone remover!?

Henkel says this it their 'Technical Service Bulletin' for Alumpirep 33:

Cleaning with TURCO ALUMIPREP 33 produces a chemically clean and corrosion free aluminum surface.
TURCO ALUMIPREP 33 can be used to deep clean and brighten an aluminum surface prior to welding, painting or to prepare the surface for a subsequent conversion coating.
Click to expand...

I thought of this process, but am not sure if I am on the right track:
- Rough cleaning of the part, removal of stickers, larger amounts of grease, oil, etc.
- Dimpling
- Give it a swim in Alumiprep
- Rinse
- Dry
- Spray P60G2

Does this make sense?

I guess that as soon as I have to manually scuff and clean each part with Acetone and with great attention to detail, it would almost be easier / faster to go the Alodine and AKZO route. Looking at this EAA video, alodining a part seems to be a very straight forward process!? http://eaavideo.org/video.aspx?v=1340416082001

What are your thoughts?
 
Etch Primer

Intersting about the Ajax wirejock and good tip thanks.

I have been just scuffing with the maroon pad, giving it a good wipe over with MEK. ( that stuff needs double gloves) and using Wattle super etch primer.
My understanding of etch primer is that it creates a chemical oxidization during the drying process like alodine does.
I have put as a test, grey duct tape on, ( Americas gift to the world) let it bake in the mild 40 deg Australian summer and ripped it off. Had to use MEK to get the residual glue off which meant re-priming the part, the Wattle primer would appear to be very satisfactory. Does scratch of cause but thats what a pressure can is for.
 
Process

Yes. Alumiprep does just what the name implies. It etches and cleans. Alodine will be happy. The use of it is a builder choice. I like the BonAmi scrub because it's not harmful to me or the environment. I swithed to BonAmi because it doesn't have the sodium hypochorite like Ajax and Comet (thanks VAF). I don't know if it or Almiprep etches sufficiently to provide a mechanical bond for primer. Some who use BonAmi combine the scuffing and cleaning by using maroon scotchbrite and Bon Ami but it may be difficult to see missed areas till it's rinsed.
I don't see a need for a solvent clean after Alumiprep or BonAmi. Both processes remove oils and aluminum oxide. It should be noted that solvents do not remove the aluminum oxide layer. Aluminum oxide starts forming imediately. You have a 2 hour window to get alodine or primer on the part. Once Alodined, the surface is chemically changed and aluminum oxide cannot form. Primer without alodine will lock out the oxygen and prevent aluminum oxide from forming.

Now to the primers. Choice is up to the builder. Basically any is better than none. Coatings need a mechanical bond to adhere to metal. Some primers claim to self etch which in theory should provide the mechanical bond. The primer needs some tooth to hang on. Most call for a sanding process but you could try your method on a sample and try the tape test after it cures. Rub a piece of Gorilla tape on the cured part. Let it sit over night in a warm place. Rip it off. If the primer comes off with it, you have your answer. I test often. Napa 7220 is supposed to self etch but it rips right off. I only use it on steel parts as a barrier from dissimilar metals. Yes, I actually prime nutplates.

Note:
I like to encourage builders to test and come to their own conclusions. You learn what works and get quality time with the gun.
I'm no expert and in fact don't alodine interior parts before primer. I do plan to alodine the exterior surfaces.
 
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Larry,

With the Bon Ami, do you just dust it on the scotchbrite, wet it, and scrub?
Then rinse/dry and prime?
 
Thank you for your detailed response, Wirejock.

After spending half the night in front of my computer, I am now leaning towards a solutions I had already discarded: PreKote, followed by AKZO epoxy primer.

I understand that the application of PreKote is very similar to the cleaning / scuffing before spraying with P60G2, but it would allow me to spray AKZO epoxy primer.


PreKote claims that...
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.
Click to expand...
http://www.pantheonchemical.com/prekote-surface-pretreatment/


I also found a letter by the EPA according to which...
The study identified that PreKote technically performed equal or better than chromate conversion coatings and eliminated or reduced a major source of pollution and hazardous waste associated with aircraft painting. The study identified that the PreKote process could eliminate the solvent wipe down, acid brightener, and sand anodizing process used in conventional paint preparation procedures. ESCA testing identified that PreKote left a molecularly thin coating on the surface which promoted adhesion of the primer coating but did not chemically alter the substrate.
Click to expand...
http://www.epa.gov/sites/production...ation_metal-finishing_epa-memo_04-01-2003.pdf

PreKote alone however does, other than Alodine, not provide any corrosion protection.
 
BonAmi

AdamB said:
Larry,

With the Bon Ami, do you just dust it on the scotchbrite, wet it, and scrub?
Then rinse/dry and prime?
Click to expand...

Yes.
Just don't do what I just did. Washed all my parts. Carried them out to the factory. Dried, marked, yada, yada, yada. Cleaned up. Walked inside and found a part laying in the rack.
Doh! Sucks getting old.:eek:
Now I have one extra for the next session.

Oliver
I like it when someone does so much research. I have no experience with Prekote. You may want to do a VAF search.
 
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Prekote then AKZO

Oliver said:
Thank you for your detailed response, Wirejock.

After spending half the night in front of my computer, I am now leaning towards a solutions I had already discarded: PreKote, followed by AKZO epoxy primer.

I understand that the application of PreKote is very similar to the cleaning / scuffing before spraying with P60G2, but it would allow me to spray AKZO epoxy primer.


PreKote claims that...

http://www.pantheonchemical.com/prekote-surface-pretreatment/


I also found a letter by the EPA according to which...

http://www.epa.gov/sites/production...ation_metal-finishing_epa-memo_04-01-2003.pdf

PreKote alone however does, other than Alodine, not provide any corrosion protection.
Click to expand...


I am currently using the PreKote/Akzo Combo. I'm still early in the build, but so far I'm happy with the results. The cured Akzo holds up very well during the occasional "rough handling" when assembling parts.
The PreKote process is not magic and does require a fair amount of elbow grease. When applied, the first light sub produces a fairly clean white foam. As the scrubbing continues, the foam turns a dark grey to black. I believe the transition from grey to black occurs when you scrub thru the alclad layer (just my guess). Per the instructions, I then apply a second coat and rinse. When rinsing, the water does not "break" off the aluminum indicating a properly cleaned part. Prekote says prime within 24 hrs, but I normally prekote in the morning and prime when parts dry later the same day. I sometimes use a heat gun on low to speed the process.

As I stated above, the process does require a fair amount of elbow grease and TIME...especially on small parts with many nooks and crannies. Because of this, I am considering a new approach. For small parts, I think I may initially clean/etch with alumiprep. Then, because I think the Prekote does provide some type of adhesion benefit for the Akzo, I will do a "light scrub" with Prekote. My theory is the Alumiprep will do most of the scrubbing I've been doing with the scotch brite pad and PreKote.
Not sure how this will work, but plan to experiment my next prime session. Open to suggestions though if anyone sees a problem with my plan:)
 
Mudfly said:
[...] The PreKote process is not magic and does require a fair amount of elbow grease. When applied, the first light sub produces a fairly clean white foam. As the scrubbing continues, the foam turns a dark grey to black. I believe the transition from grey to black occurs when you scrub thru the alclad layer (just my guess). Per the instructions, I then apply a second coat and rinse. [...] As I stated above, the process does require a fair amount of elbow grease and TIME...especially on small parts with many nooks and crannies. [...]
Click to expand...

Are you sure that you are not overdoing this? Watching their videos, I gained the impression that a quick scrub is sufficient. I understand that as soon as the foam develops, PreKode will be applied a second time. After another scrub of similar intensity, the part will be rinsed.

Their general aviation application training video: https://youtu.be/jO_mbv9Uox8

In the F-16 video it indeed appears as weather they neither spend much time on scrubbing nor that they apply much pressure: https://youtu.be/Kx3H8I0ll9Q
 
Oliver said:
Are you sure that you are not overdoing this? Watching their videos, I gained the impression that a quick scrub is sufficient. I understand that as soon as the foam develops, PreKode will be applied a second time. After another scrub of similar intensity, the part will be rinsed.

Their general aviation application training video: https://youtu.be/jO_mbv9Uox8

In the F-16 video it indeed appears as weather they neither spend much time on scrubbing nor that they apply much pressure: https://youtu.be/Kx3H8I0ll9Q
Click to expand...

It is possible that I am "overdoing this" ..I have been accused of that in the past:) The process is not bad at all on larger parts..especially flat skins like in the videos. However, my process is slowed way down when I'm working with smaller parts with several flanges and tabs. I think the hard part may be removing the the shiny/smooth alclad layer. That's what I was hoping the alumiprep might help with. I think alodine after alumiprep would be the best, but with my location I wasn't comfortable using it.
 
Oliver said:
Why would one want to remove it? It is pure Aluminum, which offers an additional layer of corrosion protection.
Click to expand...

I think there are several opinions on that. Everyones situation may be different..primer used, prep used, type part, etc.
Here's part of a a post I read from this board. This was from member MikeH.

"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."
__________________
 
AKZO is tough as nails

On my top skin, I'd maroon scotched and degreased only the area that goes under the front wind screen (ie in the cockpit) and left the outer area for my outside paint primer (whatever that will be) and didn't clean it or scotch it - and it beaded water like you would expect. Naturally when I got to shooting the Akzo, I brain farted and shot the whole top skin. The Akzo on the non scrubbed/cleaned area is wicked hard to come off. It'll have to be mechanically sanded to get it off. That's a heck of a paint bond.

YMMV.

Mani
 
Dont remove the Alclad!

You do not want to remove the Alclad...and it would take alot to do so. You just need to clean it and scuff it for the primer adhesion. Alumiprep and the likes wont eat the Alclad away, and red Scotchbrite used to scuff it wont hurt either. The pure Aluminum Alclad is the single best defense against corrosion. The edges of the metal (including holes) are where corrosion will start and do damage. I used Scotchbrite and soapy water to wash prep to a "break free" (water wont bead up) surface, dried, and alodined. You can brush alodine, wipe with a rag, ect...not always a dip. Just be sure to rinse before it dries. I primed all of my skins inner surfaces prior to installing them using a smooth foam roller and slightly thinned epoxy primer..no mess, great coverage and most that have seen it thought it was sprayed. thats how we do big jets in the hangar without a mess.
 
maniago said:
On my top skin, I'd maroon scotched and degreased only the area that goes under the front wind screen (ie in the cockpit) and left the outer area for my outside paint primer (whatever that will be) and didn't clean it or scotch it - and it beaded water like you would expect. Naturally when I got to shooting the Akzo, I brain farted and shot the whole top skin. The Akzo on the non scrubbed/cleaned area is wicked hard to come off. It'll have to be mechanically sanded to get it off. That's a heck of a paint bond.

YMMV.

Mani
Click to expand...


Yeah, that's for sure. I do the same as you; Scotchbrite/degrease and spray.

I spray up against house wrap (tough waxy paper) - sticks to that. Old outdoor aluminum table - sticks to that. Built my own DRDT frame that gets any left over spray - sticks on that too. Wood stir sticks - yep.

Now I'm NOT suggesting to not prep surfaces properly, but the more I use this stuff the less hard I am on the alclad when scrubbing with the Scothcbrite. Seems the best of both worlds is to leave as much of the alclad as possible while abrading enough for a good bond.
 
Is it necessary to degrease the al clad wing skins after the blue protective polymer film layer is removed and before etch priming? I have de-burred the sheets for my RV 12 keeping the blue polymer film on. I then removed it using gloved hands to avoid putting fingerprints on the surface.
The surface appears pristine. Degreasing agents can themselves leave a film on the surface if used. I am interested to know what other builders do.
 
There are oil residues from the mill when processed. It ships to Vans. Van applies blue plastic. The oils aren’t being removed by applying and peeling blue plastic. There’s red ink from the mill marking process that needs to be removed.

I would not prime over oil and red mill marks.
 
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FWIW...bucket of hot water, with Dawn and enough Alumiprep to make bare hands sting. Use the mix to scrub with red scotchbrite until rinse water sheets. Final rinse, dry, shoot with an epoxy primer. Simple, non-toxic, fast.
 
DanH said:
FWIW...bucket of hot water, with Dawn and enough Alumiprep to make bare hands sting. Use the mix to scrub with red scotchbrite until rinse water sheets. Final rinse, dry, shoot with an epoxy primer. Simple, non-toxic, fast.
Click to expand...
For those who have ranch hand tough skin, 1-qt alumiprep to 3-qts water is the chemistry-kid ratio for the acid bath. +10 for the Dawn detergent cleaning!
 
Dr_Dave_MT said:
For those who have ranch hand tough skin, 1-qt alumiprep to 3-qts water is the chemistry-kid ratio for the acid bath. +10 for the Dawn detergent cleaning!
Click to expand...
Or save you skin and scrub with maroon scotchbrite and Bon Ami Cleanser. Scuff and clean in one environmentally/human friendly process.
 
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