Painting aluminum: preperation details

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kmorin
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Painting aluminum: preperation details

Post by kmorin » Thu May 11, 2017 1:18 pm

I'm writing, again, about painting aluminum because I've recently been involved with a friend's airplane paint job. While a plane is mostly 2024 cladded sheet and not 50 or 60 series aluminum alloys the paint prep. process is similar and these points can help make the job easier IF (huge consideration) you're planning to use the traditional chromic acid (alodine) conversion of aluminum oxide to aluminum chromate as the primer bond layer to the underlying metal.

Let's review. #1 if any aluminum alloy is left in the air for a few seconds it forms a 'self-healing' or depth limited few molecule thick layer by chemically combining aluminum with oxygen. The result is a layer or aluminum oxide that 'seals' the underlying metal. The word seal here is used to mean that the chemical reaction to oxygen is stopped because the layer of oxidized metal does not allow further depth of reaction down into the aluminum. This attribute of the Miracle Metal is why is has commercial value- if it reacted like iron/steel/alloys of iron then; Aluminum would dissolve very quickly into a pure shape of oxide with no metal remaining.

Steel slowly continues to oxidize; aluminum does not. Aluminum oxide forms in a few seconds and stops- adding a small percentage to that layer over time (many years) but remaining very thin layer. This layer melts at temperatures much higher than the underlying metal- so it needs to be taken into account when welding (various methods) but for consideration of gluing, painting, or coating aluminum this oxide film is not a very good surface to 'stick too'.

#2 Paint sticks to aluminum by one of two methods or a combination of the two methods. The first method is mechanical- where the surface of the metal is roughed up- this term rough is only taken to mean at a microscopic level. The roughness needed to bond paint is still pretty much smooth to the touch compared to a concrete block - say. If the surface is blasted by high speed particles (blasting w/ different media) the surface will become rough - this process is called creating an anchor pattern.

Think of the Velcro product as the surface of the metal that has been blasted- when that surface has been visually magnified many times so the anchor pattern can be seen. While that is not a perfect example- the idea being illustrated it the fact that even if the paint will not 'grab' chemically to the metals' surface- if the paint were to flood all the little tiny valleys and dry on the ridges too; that paint film with have a grip on the metal surface- which we refer to as mechanical adhesion.

There is another method of 'grabbing' the metals' surface and that it to chemically treat the surface with a very large crystalline (viewed in a microscope) film that gives an anchor pattern for the paint's primer layer. This method is to chemically convert the aluminum oxide to aluminum chromate and that is done by a few steps we've discussed before but will review here, as well.

The chemical conversion method chemically bonds to the aluminum without aluminum oxide- and the paint sticks well to the chromate converted surfaces- so this method is an alternative to blasting.

The ultimate method would be to blast, (blast media is not being discussed or explored) then follow all the steps to convert the surface to aluminum chromate and then primer and paint. However, it may be obvious this method, while the most effective paint preparation is also the most expensive- by far.

#3 the steps to chemically convert aluminum surfaces from aluminum oxide to aluminum chromate are; A) wash and clean with soap and water so all contaminants are removed from the metal. B) using an acid, cover the metal and allow the acid to 'etch' or chemically strip (combine with) the aluminum oxide layer; C) after the acid acts on the surfaces for a few minutes rinse continually with tap water, diluting the acid to neutral ph, flushing any remaining acid off the metal, cleaning the metal with a film of water and MOSTest important SEALing the air away from the aluminum...D) the coating the water covered aluminum with a chromic acid solution (hexavalent chromium in a mild acid) to allow the chrome to react with the aluminum BEFOre the oxygen can.

The result of this simple but potentially messy wetting, rinsing and re-coating process is a layer of aluminum chromate bonded to the aluminum.

This film will hold most paints well and is chemically bonded to the underlying metal so the "paint sticks" to the metal by both a mechanical adhesion from the primer film layer to the chromate and under that chromate a chemical bond to the metal's formerly oxygen linked -now chromium linked- molecules.

I'll expand on this a bit, and hopefully Chaps, our resident painting expert, will add points of clarification and other facts that pertain to this process.

Cheers,
Kevin Morin
Kenai, AK
kmorin

kmorin
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Re: Painting aluminum: preperation details

Post by kmorin » Thu May 11, 2017 4:07 pm

First time aluminum cleaners- owners, contractors, friends of owners who are roped into helping (!) all need to understand this process (called here; Traditional Aluminum Prep) includes potential hazards to your health, that need to be avoided by using PPE.

PPE stands for personal protective equipment and includes devices that protect the bodies' five senses and covering(s) from those hazards. However, like driving every day on highways in huge metal boxes, where the converging speeds exceed 100 mph, hazards are just problems to be solved. PPE is designed to keep the hazards away from you- so you avoid bodily damage from these hazards- mainly chemical in nature.

Breathing toxic or chemical laden fumes is a hazard- but can easily be avoided by a half face- air purifying respirator; that is properly sealed to the face- without facial hair leaking of the seals.

Having the skin in contact with acids or hot fluids can cause not only chemical burns but potentially skin absorption of dangerous chemicals. So gloves, rain suits, face masks, splash goggles, and rubber boots are all considered part of the normal PPE to handle and use strong acids or chrome solutions.

The 'skin' of your eyes; corneas or other parts is covered above but the goggles are often used to keep the eyes out of vapor streams like sprayed acids or hot rinse water splashes. You might heal your hide on the backs of your hands if you get a chemical burn but--- the eyes' -- maybe not so much?

While all this protective gear may sound like the world is coming to an end if you use acid etch/rinse/alodine aluminum conversion? that is an exaggeration for the purpose of instilling caution. What does a car coming at you- 4' to your left look like? Well its a potential collision with a tonne of metal closing at 100 mph (relative) but you do it all day- every day, and now that I mention it- you haven't thought much about it since you were 16 and got a license.

The same is true of these chemicals- they're what's needed to do the job and with normal rain suit (oil skins) and some eye, breathing and hand protection the materials are safer than the highway as more people die every day from highway collisions than HAVE EVER died of etching their own boat!

Last to consider in this part of the description is the materials themselves- and what happens to them when they're 'used' and have been rinsed off? First let's do the etching acid as the easiest of the two haz. compounds, the chromic acid follows somewhat but has the added metal in solution.

Acid is dangerous or hazardous in direct proportion to dilution or concentration (two ways of saying the same thing; where dilution/concentrations are two sides of a verbal coin) so; if you have one drop of 'pure' acid in swimming pool- you can't even detect that there is any acid and the pool water won't 'burn' or corrode either your skin or metal surfaces.

However, if you have drop of 'pure' (pure is not defined here intentionally) acid of some types- that drop can be let loose over your palm and "drop through" like there was no palm there! I'm using an extremely strong acid reference for the purpose of showing the vast range of acidic reactions- to show that concentration is what makes any reactive chemical dangerous. We often hear 'strong' and 'weak' used to describe concentrations of acids- while not exact that can be helpful to understand that vinegar is a mild- not strong and not weak acid.

The acids that will chemically etch/combine/lift/remove the aluminum oxide layer can range from household vinegar to industrial products that are intended for use by trained workers. However, ALLLLLL... any, every type and kind of acid; can be diluted with water and become less concentrated (weaker) and therefore less dangerous as the reactivity of the acid has been diluted and the relative 'strength' has been reduced.

If you pour milk into coffee- for a long enough time; there becomes more milk than coffee. If you pour water into acid for a long enough time/volume the chemical result is more water than acid. So one of the steps in this process to clean aluminum is to dilute the acid after its combined with the aluminum oxide layer- and simply change the acidity to nearly water.

Therefore.... if you have a hazardous acid and you dilute it at 1,000 or 10,000 to one by volume (spraying a garden hose on a cup of acid, flooding an sheet of aluminum is like this) then the acid is no longer 'acidic' and becomes "water" for disposal purposes.

Disposal of acids is generally regulated by the concentration of acid- if the acidic liquid is only 0.001% acidic- or some other tiny amount "acidic"; the regulations do not address disposal. It can be run into drains, leach into the yard, driveway, or back lot- without risk or harm since the dilution is so great the concentration is so low the liquid is no longer -acidic.

So, if you rinse acid off a boat, and then drain the rinse into the ground around the work- the risk of being in violation of disposal of hazardous fluids is minimal due to dilution. Then next time it rains the dilution will go up and the concentration will become rapidly undetectable. Remember once its diluted to vinegar there can't be any risk to the environment or we couldn't cook with and eat vinegar level acids in our homes.

All that is true of chromic acids' (alodine's) contents to some degree a teaspoon of this acid in the swimming pool won't make poison compared to the chlorine used to purify the water. But dilution is again key to any regulatory provisions for use and disposal. What is important in either case if for a boat owner who plans to use the traditional methods of painting adhesion to his aluminum boat to be aware of the need to dilute these chemicals after they're rinsed off the job.

However in the case of Chromic Acid (alodine and similar compounds) there remains some 'heavy metal' and that even diluted can be subject to regulation- you need to check with the various manufacturer's recommended handling procedures to be sure you know what you should do in your area? I'm shy of suggesting you contact the (so called) envir'nm'ntal agencies as they tend to be run by ignorant hysterics who cannot make rational decisions but react to 'trigger words' and become abusive of other humans as a general practice. (Witness the people in Idaho who were nearly financially ruined by having a mud puddle from a rain storm in their proposed new house's backyard?)

Keep these compounds off you, use them carefully and make sure they're both 'super' diluted so they pose no concentration risk to the water shed or environment and you're safe using them both to create a conversion surface film for primer that will adhere well for a long time.

More to follow.

Cheers,
Kevin Morin
Kenai, AK
kmorin

kmorin
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Re: Painting aluminum: preperation details

Post by kmorin » Thu May 11, 2017 8:38 pm

Are there simpler ways to get paint to stick to aluminum?- you're asking after reading these posts above about the steps taken (regularly) to clean oxide off of aluminum and convert to aluminum chromate instead of an aluminum oxide film on which to attach primer for painting.

Media blasting is a way to put an anchor pattern on the metal that will grip paint- or perhaps better said- paint will grip blasted aluminum (we're kind of confining ourselves to 5000 and 6000 series even if my recent 2024 series aircraft aluminum was the inspiration for this series of posts) by drying entwined in the ragged edges of peaks and valleys left by the impact of a high speed semi-hard particle with the softer metal.

There are several different groups of particles that are "soft enough" (fracture before they actually drill holes in the metal!) but still have little grains or particles that bombard the surface and create the anchor pattern on which primer paint will hold to the metal.

Steel, and other harder metals are blasted with much harder materials than aluminum -typically- so being aware of what media (name for the stuff you send down the blasting nozzle onto the metal) is important- critically important.

If you rent sand blasting equipment- the rental outfit will often offer to sell media/sand/blasting agent for the application you have. So if you are told that garnet sand - hard and used on steel- is "fine for aluminum" then I'd suggest you find another expert. Softer sands like silica and even filter sand (sometimes called "sugar sand") are usually softer and therefore will create their impact site on the metal but aren't as likely to dig a hole or thin the metal if you hold the hose in one place too long?

For those who'd like to understand how to sand blast an airplane's surfaces? Soda blasting http://www.sodablastingmn.com/html/Before_&_After.htm here's a typical link that will lead to others if you're interested. This method is kind of expensive by comparison to paint stripper and old retired welders scrubbing rivets using paint stripper!

Just a reminder that after you blast a boat- it is the best (but not absolutely needed) paint preparation to go ahead and use and acid etch and rinse then coat with alodine- getting both a blasted anchor pattern and a chemical bond to the aluminum as well for the primer.

Unfortunately sand blasting as a whole project is not easily done with a small HomeTrainStation brand compressor that runs on 117VAC! This method of cleaning/blasting/prepping a boat will require and engine driven compressor that can delivery hundreds of cubic feet per minute of 'free air' at substantial pressure at the nozzle. So when the term sand blasting or media blasting is used- please make sure you understand that usually implies a hundreds of dollars per day rental and fuel to run the large trailer born compressor; a very heavy duty hose and full body protection and a "supplied air" breathing system for the blaster.

Do NOT, under any circumstances plan to do this work by yourself. YOU MUST have, and attendant/helper/rigger at the very minimum- other wise- you're asking for an accident to happen. The hose has to be turned on and off by the attendant, the safety of the blaster is usually in the hands of the attendant and there is no real safe short cut in this matter.

We've reviewed some of the main points of painting prep both chemical surface conversion and mechanical surface preparation- the purpose has been to collect in one thread an approximate summary of what is needed to paint and aluminum boat - where the paint will stay on!

More to follow.

Cheers,
Kevin Morin
Kenai, AK
kmorin

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Re: Painting aluminum: preperation details

Post by Chaps » Thu May 11, 2017 11:58 pm

Good info there Kevin, thanks for the write-up.

Am I to understand, then, that when choosing to prep aluminum with chemical treatments that we are simply developing a chemically induced anchor pattern on the metal? And that a chromate layer provides a better anchor point for coatings than acid etched aluminum alone? Why is it critical to apply the alodine in tight sequence to the acid etch? Will the chromate layer only form on raw, unoxidized aluminum?

Generally speaking the coating manufacturers recommend either prepping aluminum chemically (per your instructions) or media blasting or, least favorable, machine grinding/sanding. In my many discussions with paint reps they all say blasting is best and assuming primers can be applied before the surfaces become contaminated no further chemical etch or conversion is necessary or measurably helpful. The profile created on the surface by blasting is sufficient in spite of what degree of oxidation occurs between the time of the blast and the time of first application of primer (within reason).

Personally I can't imagine attempting to acid etch and chromate convert an assembled boat of any size and in the case of doing bottom jobs applying acids and other chemicals overhead would be futile (and dangerous). Don't get me wrong, blasting brings its own issues, some of which you cited above but overall it's a better, faster process that is easier to control and far less hazardous to health and environment.

This is a recent job in my shop. Hull had never been painted and bottom was heavily oxidized, pic #1. In fact I doubt an acid etch with typical solutions would have had sufficient impact on this boat without several applications. As can be seen in pic #2 the bottom after sandblasting could hardly be any better prepared for coatings. Its is profiled to a surface that feels to the touch like 120 sandpaper, it is clean, dry and raw. Pic #3 shows the boat 3 hours later with a solid initial coat of epoxy primer. During application you could feel the aluminum literally sucking the epoxy out of the roller, it just doesn't get any better than that and I seriously doubt that a small accumulation of oxide on that blasted surface will ever be a factor that could lead to coating failure.
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kmorin
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Re: Painting aluminum: preperation details

Post by kmorin » Sun May 14, 2017 12:11 am

Chaps wrote:
Thu May 11, 2017 11:58 pm
Am I to understand, then, that when choosing to prep aluminum with chemical treatments that we are simply developing a chemically induced anchor pattern on the metal?
Yes, but... the chemical, chromic acid (alodine) combines/bonds/molecular level joins with pure (alloyed but not oxidized) aluminum to form aluminum chromate. This film of molecules are bonded chemically to the aluminum- paint can't bond at a molecular level but... since these molecules are highly rough (microscopically) crystalline/jagged/irregular surface film the top side allows paint film to 'freeze' or dry into a bond to that rough surface.

Alone the aluminum chromate is a good corrosion inhibitor- not much in terms or scratch resistance because its so thin... but good chemically.
Chaps wrote:
Thu May 11, 2017 11:58 pm
And that a chromate layer provides a better anchor point for coatings than acid etched aluminum alone?
Yes Way better than just etching alone. The reason is the chemical bond to the aluminum is coupled with a rough surface not available from aluminum oxide film that forms when the metal is allowed to have air for 3-5 seconds.
Chaps wrote:
Thu May 11, 2017 11:58 pm
Why is it critical to apply the alodine in tight sequence to the acid etch?
Aluminum will oxidize for a mill deep in a few seconds. This film is best avoided to give the alodine the most un-contested access to the bare aluminum (very reactive) molecules. So putting the alodine on while the etched (oxide stripped) metal is covered with water excluding oxygen for the most part provides the chromate formation the most effective circumstance to 'work'.
Chaps wrote:
Thu May 11, 2017 11:58 pm
Will the chromate layer only form on raw, unoxidized aluminum?
No, pure alodine or chromic acid will bond sort of randomly to oxidized aluminum -but at 50$ a gallon for acid or chromic acid(?) local prices- a gallon of alodine will go 10-20x as far if it is applied to the freshly etched an unoxidized aluminum while its still wet. The water, even with some oxygen entrained, simply excludes the 21% oxygen atmosphere and gives the aluminum chromate first chance to form- preventing aluminum oxide's formation until the surface molecules of metal are bonded to the chrome and oxygen instead of just pure oxygen.

Chaps, I agree that Michelangelo acid and alodine jobs would be the pits- or maybe I'd wish I had a pit to hide in if I had to do one! But, the methods we evolved allowed the acid to be sprayed in small patches- 2' wide and wing width. The left to soak or even abraded (in full rain gear) with Scotchbrite (tm) while cleaning the surfaces. Then we flooded the areas with water and kept flooding while we used a spray bottle of alodine on the wetted surface.

We found we could do 4' wide wing 20' long in less than three hours on both sides with two men working. So by keeping the patches very small, the rinse continually on the work (raining on you too) and the alodine coming immediately on the rinse- we got even coverage, controlled coloration, good layer of uniform prep.

The paint films I think you're using and the viscosity being used on planes- are significantly different? I'm not sure on that but the wt of paint is actually a big deal. My primer coats on boats - in the past - were probably 4 to 8 X thicker than this final film (primer PLUS two top coats). The paint used was the PPG system of acrylics- but I didn't shoot this paint- I just prepped the metal and don't know which series of paints were used.

If I had a skiff to do, I'd roll her side to side at the chine to get the bottom converted, last time I did this I lifted the bow and did the bottom hanging in an overhead lift. But I've only painted hulls, bottom coat and topsides/interior using an acid etch and alodine conversion.

I do realize the huge anchor pattern depth by blasting provides an equivalent grip of the primer to the metal. My point here was more to discuss an owner's do it your self project- what it would take to get a boat ready to take paint- and sand blasting is much more equipment intensive - cost intensive and requires more set up and demob as well as more facilities to do the job well.

Hope this puts the posts in context?

Cheers,
Kevin Morin
Kenai, AK
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Re: Painting aluminum: preperation details

Post by Chaps » Tue May 16, 2017 2:22 am

Good stuff, thanks
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