Question- Powder Coating of aircraft parts

Powder coating has become a common finish for many non-aviation applications and a few aviation items. That probably isn't suprising, since many of us have likely seen powder coating peform well in challenging environments that would have removed other types of coatings quickly. The longevity, relatively low cost, and ease of application may be arguements for it's use, however there are a few concerns.

Once the part is put in place, say it is chipped at a later time. How is this part repaired? Is it removed, stripped, and recoated? Or is the affected area sanded and painted? My old Cessna repair manual says nothing about a powder coating as a refinishing method. Speaking generally, are repair methods limited to what the repair manual includes and what is described within AC 43.13?

Temperature may be a concern. Some aircraft parts are heat treated. It may be difficult to ascertain whether or not the common 400 degree powder coat bake will have any metalurgic effects, since we often don't have the engineering data for the part in question and most of us aren't metalurgists. How do we know for sure? For example, the Cessna Wittman type main landing gear. It would be fantastic to have the durability of a powder coat, but can we safely and legally apply powder to the MLG?

One other concern I've heard expressed is that powder coating may form such a tough, impervious and flexible membrane that the base material may be cracking or corroding, but problems are hidden by the powder coat. Is this a realistic concern?

First person experiences, peer reviewed studies, sharing of manufacturer guidance or FAA guidance would all be of interest.

There was an NTSB report last year concerning the crash of a Safari homebuilt helicopter. The cause was a powder-coated cyclic control stick fracturing.
On a personal note; I have seen several aluminum rims that were powder coated fracture at the dragstrip. Some people have good luck with the process and some don't. In any case it is not something I would like to have in the back of my mind while I'm flying my plane down the backside of a loop...

Chris

The common routine for Cad plating on steel parts require baking after plating at 400 degrees, so I wouldn't worry about powder coating them, but heat treated aluminum, I wouldn't bake at any temps over 250 degrees. You would be required to be very well versed in the treatment process on the aluminum material, and stay well below the critical temps.
I know there are powders that will bake at 200 degrees.
As for the cracking problem, the new URA paint systems are as thick, and as pliable, so what's the difference?

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Originally Posted by Hiperbiper

There was an NTSB report last year concerning the crash of a Safari homebuilt helicopter. The cause was a powder-coated cyclic control stick fracturing. Chris

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Chris
Do you have a link? I'm having no luck finding it. Closest I could find was a control rod failure, but wasn't powder coated......

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Originally Posted by Tom Downey

new URA paint systems are as thick, and as pliable, so what's the difference?

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Tom,
I've googled URA paint systems and not getting much. Is it a propriatary product or a general term? I did find this http://www.hentzen.com/index.php/pro...rands/ura-zen/

I'm hearing 3 mils as a light coat and 5 mils as heavy from the local powder coating shops. I don't think there is much of an argument that powder is going to always be thicker than non-powder techniques.

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Originally Posted by nomocom

Tom,
I've googled URA paint systems and not getting much. Is it a propriatary product or a general term? I did find this http://www.hentzen.com/index.php/pro...rands/ura-zen/


I'm hearing 3 mils as a light coat and 5 mils as heavy from the local powder coating shops. I don't think there is much of an argument that powder is going to always be thicker than non-powder techniques.

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It's just a nomer for any ____urethane product.

Usually steel parts get powder coated. These days you will find many steel tube airframes, both kits and factory airplanes, that are powder coated. Engine mounts are powder coated. Steel heat treating involves 1000+ F temps so the temps used for powder coating are not even close to being a factor with steel. I would not worry at all about putting a powder coat finish on a steel part, including a landing gear leg.

Aluminum is more commonly anodized or painted. Aluminum heat treating still involves temps up above the 700 F range, so powder coat temps should be OK. That said, Aluminum assemblies are more likely to involve a number of small parts bolted or screwed together. Powder coating is a thicker and weightier finish than paint. When we build something out of aluminum, we are looking to make it light. A paint finish is lighter.

So why powder coat a part? Two reasons. If a part is exposed to a lot of heat, like an engine mount, powder coat handles it better. The thicker coating resists erosion of various sorts better than paint. Second, when you are finishing a spidery tubular structure, its is hard to spray paint without wasting a lot. A smaller gun or even an airbrush will let you put more on tubes and less into the air, but it is very labor intensive. So the manner in which powder coat is applied avoids the labor of spray painting and is more efficient in terms of wasted material.

I shoot polyurethane paint. If you can do it outside in open air it is not hard to do. Wear a good respirator as breathing the stuff can kill you in short order. Do NOT do it inside unless you have a real paint booth. The local automotive paint distributor can sell you an entry level spray gun and the stuff that you need to get started. Or some EAA chapters have a paint booth that they share. It is not hard to learn to do.

You can get a lot more interesting colors in paint.

I will note that I never thought that I would own 6,000 grit sand paper. I painted a fuel tank and started wet sanding and was not happy with what I saw until it really shined. My spouse says it is the prettiest fuel tank on the planet that no one will ever see.

And if you learn to shoot real paint, you can update your kid's wagon, add some stripes to your lawn tractor, and generally do stuff other than airplanes.

Best of luck,

Wes
N78PS

One reason for not powder coating steel is it tends to hide cracks. It seems like the ideal coating for an engine mount but is really one of the worst. I've cut into tubing that was rusted under the coating but didn't show any surface defects.

Great for cosmetics, poor for structures subjected to stress.

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Originally Posted by turtle

One reason for not powder coating steel is it tends to hide cracks. It seems like the ideal coating for an engine mount but is really one of the worst. I've cut into tubing that was rusted under the coating but didn't show any surface defects.

Great for cosmetics, poor for structures subjected to stress.

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You have a much greater chance to see corrosion on a painted steel than you will on powder coated simply the powder coaters prepare the surface better.

I will offer the suggestion that the above statement is contradictory. Great prep results in a long lasting finish that resists the elements.

What I think that many folks miss is that tubing has two sides, an inside and an outside, and we have a history of neglecting the inside. A lot of tubing corrosion literally surfaces after starting from the inside of the tube. You can find AD's that require regular testing of engine mount tubing with a punch followed by filling and draining the tubing with preservative oil. Those engine mounts are failing because the manufacturer welded up the assembly, painted the outside, and left the inside unprotected. It is worth your time to drill holes before welding up the joints so that all of the insides of the tubing are connected. Then you can flush some preserative oil through the inside after painting the outside. Protection of the inside avoids problems appearing on the outside later.

Best of luck,

Wes

When the tube rusts from the inside out the surface perp doesn't mean much. The most frequent failure of an engine mount is because some one placed a zip tie on it and allowed it to wear a groove in it. Powder coating will fail in this case too, but it takes longer.

The fuselages and tubular parts on the Maules and the Ayres Thrush Ag planes are powder coated.

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Originally Posted by Hiperbiper

There was an NTSB report last year concerning the crash of a Safari homebuilt helicopter. The cause was a powder-coated cyclic control stick fracturing.
On a personal note; I have seen several aluminum rims that were powder coated fracture at the dragstrip. Some people have good luck with the process and some don't. In any case it is not something I would like to have in the back of my mind while I'm flying my plane down the backside of a loop...

Chris

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Chris, is Gilbert Goodlett's accident the one you are thinking of? I attached it below. It was a non-fatal Safari accident in 2007, and it was apparently due to anodizing a control rod, not powdercoating.

Jim

Attachment 1675

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Originally Posted by Jim Hann

Chris, is Gilbert Goodlett's accident the one you are thinking of? I attached it below. It was a non-fatal Safari accident in 2007, and it was apparently due to anodizing a control rod, not powdercoating.

Jim

Attachment 1675

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Jim; you are correct; that is the crash I was (wrongly) refering to...

My apologies to all the powder-coaters I might have impuned.

Chris

This sounds like an interesting discussion -
Powder coating seems to be another topic where many people have a personal preference. Sort of like which finish paint to use, what type of engine oil, etc... So, I will add my two cents worth:
1) Inspections - Coming from the perspective of performing annual inspections and such, I would rather have a coat of paint instead of powder coat for the reasons listed previously by others.
2) Corrosion - the engineering department I work for recently completed extensive analysis (as in very expensive and time consuming) of powder coating with regard to corrosion, with the final result being that we will not powder coat over bare metal. Powder coating offers a very thick coating which provides good barrier protection, but there is more to an effective corrosion resistant coating than that. The good thing is that most aircraft, except for seaplanes in salt water, don't see the environment we were testing for. We consider powder coat as a top coat only (like a coat of enamel), to be used over a corrosion resistant primer, which is on top of the proper conversion coating for the particular base metal being used.
3) Fatigue - we are not aware of any hydrogen embrittlement issues, and would not think the baking temperatures would be an issue except for some heat treated parts.
4) Repairability - a coat of paint wins out for ease of repair.

Randy

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Originally Posted by Hiperbiper

Jim; you are correct; that is the crash I was (wrongly) refering to...

My apologies to all the powder-coaters I might have impuned.

Chris

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No worries Chris, just want accurate info out there!

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Originally Posted by R. Novak

This sounds like an interesting discussion -
Powder coating seems to be another topic where many people have a personal preference. Sort of like which finish paint to use, what type of engine oil, etc... So, I will add my two cents worth:
1) Inspections - Coming from the perspective of performing annual inspections and such, I would rather have a coat of paint instead of powder coat for the reasons listed previously by others.
2) Corrosion - the engineering department I work for recently completed extensive analysis (as in very expensive and time consuming) of powder coating with regard to corrosion, with the final result being that we will not powder coat over bare metal. Powder coating offers a very thick coating which provides good barrier protection, but there is more to an effective corrosion resistant coating than that. The good thing is that most aircraft, except for seaplanes in salt water, don't see the environment we were testing for. We consider powder coat as a top coat only (like a coat of enamel), to be used over a corrosion resistant primer, which is on top of the proper conversion coating for the particular base metal being used.
3) Fatigue - we are not aware of any hydrogen embrittlement issues, and would not think the baking temperatures would be an issue except for some heat treated parts.
4) Repairability - a coat of paint wins out for ease of repair.

Randy

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I agree whether to pow coat or not seems be driven by personal preference. Personal preference is great when it comes to colors of favorite foods, but when we are talking about a maintenance procedure, I'd like to think we can come up with more. I suppose that is why I started this thread. Along those lines, could you get some underlying data from the "extensive analysis". Without that, we are left trying to piece together the data and the logic train- an effort I'm lousy at and when you read the below, you can see I failed miserably, as I come to different conclusions than the engineering department.

Too Thick? It was suggested earlier in the thread that new polyurathanes can be laid down every bit as thick. I seemed to get verification of that yesterday. The local paint shop suggests 3 coats of polyurathane on top of 2 coats of epoxy primer. Now how thick that will be- not sure, but I'd think similar enough I have to toss the "thickness" argument. Should we only be painting thin, so we can see what is going on with the base metal?

The local powder shop does coatings for lots of different industries, one being whitewater equipment. These parts get banged against rocks, other equipment, tossed in the back of a trucks and bounced down backroads. He had some rafting frame tubes lying around. The superviser took a spare 1 inch tube that had been coated and as we toured the process, he wacked steel beams and other hard objects near our path. The result, a dented tube, but the coating stuck right to it, it didn't scrap off or peel. He explained not only does the powder cross link to itself, it also chemically bonds to the steel- which is exactly what you want if you are trying to prevent corrosion, no? What is a primer needed for when the powder has locked onto the steel surface?

3) Hydrongen embrittlement? Maybe a concern about paint removal on main landing gear? In relation to MLG, IMHO, there has been a generalized reaction that fails to consider stripper type. There are some newer acid strippers on the market that should NOT be used (H+ donors) but you can figure out which ones those are by reading the label or the MSDS. I stuck to the old style methylene chloride (basic pH) that has been used on airplanes for ages- aluminum and steel. Furthermoe, if a part is suspected of hydrogen embrittlement, one of the treatments is an oven to drive out the hydrogen, so at this point I see no connection between HE and powder coating. If there is one, please educate us.

4) Airframes based out of Alaska produces powder coated PMA'ed Piper MLG. I've chatted with one of their engineers. Continued airworthiness consist of inspection, then repair of any uncoated metal. Thing is, all that repair consists of is painting, rough it up, and throw some paint on. This touchup should be less frequent with powder coat, because it takes more abuse for it to come off. It would seem to follow that powder coating would win here, since it can have the same method of repair, but can be expected to need repaired less often.


Wes had a good point out earlier about the importance of maintaining the inside of the tube as well. Thanks for that. Do we agree outside versus inside condition and maintenance are independent of one another? Obviously both important! Turtle stated he had cut into tubing that looked good on the outside, but was rotten on the inside. Playing devil's advocate here, that is not a phenomenon unique to powder. Go on a used car lot and look for some nicely painted layers of rust. If you look long enough, you'll find some.

If we want to detect inside corrosion or cracks the earliest possible (without looking inside) should we not keep the outside bare? That would be silly. OK, maybe an eddy current inspection? That would be reasonable. What might shed some light on this concern is head to head testing between different coatings on steel that was forced to crack. Would the crack hide longer on a PC MLG or a polyurathane MLG? Need test results!

As the OP, I'd like to thank those that have contributed. Lacking any data otherwise, I'm heading in the direction of believing the only problem with powder coating MLG is that it is new. If I'm off track here, beat me up. I'll take it.

I'd like to mention a point that you may have missed. many old structures have linseed oil in the tubes, what happens to it when you heat the structure to 400 degrees?

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Originally Posted by nomocom

Wes had a good point out earlier about the importance of maintaining the inside of the tube as well. Thanks for that. Do we agree outside versus inside condition and maintenance are independent of one another? Obviously both important! Turtle stated he had cut into tubing that looked good on the outside, but was rotten on the inside. Playing devil's advocate here, that is not a phenomenon unique to powder. Go on a used car lot and look for some nicely painted layers of rust. If you look long enough, you'll find some.

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I didn't say the inside was rotten. I said the metal under the powder was rusted. The outside of the tube looked perfect, no sign of bubbling or rust bleeding through the finish. The cutting chipped the powder and we found rust underneath. Paint over rusty tubing and it will be bubbling withing months, if not weeks. Powder over rust and it can look good for years. Would you rather find the defect sooner or later?

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Originally Posted by turtle

I didn't say the inside was rotten. I said the metal under the powder was rusted. The outside of the tube looked perfect, no sign of bubbling or rust bleeding through the finish. The cutting chipped the powder and we found rust underneath. Paint over rusty tubing and it will be bubbling withing months, if not weeks. Powder over rust and it can look good for years. Would you rather find the defect sooner or later?

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Thanks for the clarification. Is the history of the piece in question known?

A good surface prep followed by an appropriate coating- powder or not, is going to prevent surface rust. I suppose I'm assuming here that we are talking about honest people performing careful surface prep and timely application of a protective coating. If that isn't the case (paint or powdering over rust) then all bets are off. Yes, powder would interfere with inspection, so would paint, mud, grease.....

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Originally Posted by Tom Downey

I'd like to mention a point that you may have missed. many old structures have linseed oil in the tubes, what happens to it when you heat the structure to 400 degrees?

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That is a very good point. Boiling off the protective oil, doesn't sound like a good thing. Someone with old tube experience please speak up. Vegetable oils degrade over time, free fatty acids form, especially with oxygen and heat. Maybe old vented tubes would have fatty residues (grunge) and unvented tubes that were opened up might have surprisingly fresh oil.

Cessna flat spring gear is not hollow, so no factor there.

The answer is that you put it back. You can buy linseed oil from AC Spruce, pour it in, spin the structure around to slosh, and drain. There are Airworthiness Directives that require that you do this periodically. Not rocket science.

Linseed oil is pretty benign stuff. If you have ever overheated your frying pan you know that vegetable oil chars. But its not corrosive in that state and only adds flavor to the next meal if you choose to leave it there.

For what its worth, you can bead blast the tubing and use an airbrush, or a hand brush, to epoxy prime and declare success. That finish will be lighter than powder coating. For a fuselage where you will be gluing fabric on and not generally subjecting the tubing to heat and abrasion, powder coating is gross overkill but it looks nice. In a volume manufacturing line it costs less. But for the individual builder or restorer it is not needed aft of the firewall.

Best of luck,

Wes
N78PS

To answer your questions about the engineering data - our testing focused on adhesion and corrosion resistance. The testing used was thermal cycling and salt spray testing. Adhesion was deemed to be not as effective as a traditional paint/resin type of coating bonding to the base metal with a conversion coating process. The result being that under adverse corrosive conditions the corrosion would creep under the powder coating - just like filform corrosion - failing whatever bond strength did exist.

In the end, our specification for coatings treats powder coat like any other top coat. We may use it, but not without the proper metal preparation and primer.

As I mentioned earlier- with the environment you would likely be operating from with an airplane - you will probably have nothing to worry about. Personally, I don't care for it but that is my personal preference. My only purpose was to offer some additional information.

As to your response about embrittlement - as I mentioned originally - we had no concerns.

In the end, it's whatever you are comfortable with. Good luck with your project.

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Originally Posted by WLIU

The answer is that you put it back. You can buy linseed oil from AC Spruce, pour it in, spin the structure around to slosh, and drain. There are Airworthiness Directives that require that you do this periodically. Not rocket science.

Linseed oil is pretty benign stuff. If you have ever overheated your frying pan you know that vegetable oil chars. But its not corrosive in that state and only adds flavor to the next meal if you choose to leave it there.

For what its worth, you can bead blast the tubing and use an airbrush, or a hand brush, to epoxy prime and declare success. That finish will be lighter than powder coating. For a fuselage where you will be gluing fabric on and not generally subjecting the tubing to heat and abrasion, powder coating is gross overkill but it looks nice. In a volume manufacturing line it costs less. But for the individual builder or restorer it is not needed aft of the firewall.

Best of luck,

Wes
N78PS

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My concern wasn't how to replace the linseed oil, as much as it was with the linseed oil blowing out, and destroying all the other parts being baked. I'll bet the oven operator, will be a tad upset when that happens.

Lacking approval or guidance from Cessna and the FAA on powder coating Cessna spring gear, I decided on polyurathane base and clear coat system. It looks very nice. We shall see for how long.

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Originally Posted by nomocom

Lacking approval or guidance from Cessna and the FAA on powder coating Cessna spring gear, I decided on polyurathane base and clear coat system. It looks very nice. We shall see for how long.

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I think you'll be happy with it. I have in the past filled the relatively rough surface of spring steel gear with epoxy and microspheres before painting. Makes a slick surface that last a long time.

I know this thread is some months old now, but revisiting this question with some finishing people leaves it still an open issue for me. We know that poor surface prep sets up an immediate recipe for the failure mode turtle mentions -- nice smooth powder coat over rust, that's only evident when a chunk of it falls off or is chipped off. We really don't want any rust on our tube structure, and we want to see it if there is any.

Apparently the preferred pretreatment for steel is a zinc phosphate conversion coating such as MIL-DTL-16232G Type Z Class 3 or 4. This is a multi-step hot process typically done in a plating shop. How do we know that a "powder coated" airframe or engine mount has been processed this way before coating?

Go find a local shop that does powder coating and talk to them. They want happy customers. And they want repeat business from their commercial accounts.

Sounds like you are in analysis paralysis. Don't let perfect prevent you from getting to good enough. Most of us won't be subjecting our hand built parts to hours of salt spray and other rapid aging influences during the course of our flying. If you find a moderately competent shop you should have no problems.

Best of luck,

Wes
N78PS

In order to have corrosion starting under the powder coating the electrolyte must find a way in. if properly done that will not happen.
all new outboard motors are now powder coated, that must tell you somptin.

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Originally Posted by Tom Downey

In order to have corrosion starting under the powder coating the electrolyte must find a way in. if properly done that will not happen.
all new outboard motors are now powder coated, that must tell you somptin.

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Tom,
Yes they are powdercoated, as a topcoat. For both iron and aluminum based materials the key to corrosion control is in the conversion coating and the primer system. Without the conversion and primer, the corrosion control is worse than a traditional liquid enamel ( cross linked or not ) due to the non permeability of the material accelerating corrosion. The reason for the change to powder was not one of superior performance, but one of enviromental and financial benefit for large scale production without a degredation in quality. From a pure performance perspective, you would be light years ahead to prime a fuselage truss with an epoxy liquid, vs just using a single stage powder.

No debate on the prep work on any material for any coatings. Wet coatings are not a cure for poor prep work.

Ura type powders are far superior to any wet coatings. They will resist scratching and maintain their shine better than any wet coatings, the limiting factor in using powders is the fact we must place them in a 400+ degree heat to cure the powder. Many items can not with stand that heat, linseed oil treated tubing being one of them heat treated aluminum being another. ( and there are others)

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Originally Posted by Tom Downey

No debate on the prep work on any material for any coatings. Wet coatings are not a cure for poor prep work.

Ura type powders are far superior to any wet coatings. They will resist scratching and maintain their shine better than any wet coatings, the limiting factor in using powders is the fact we must place them in a 400+ degree heat to cure the powder. Many items can not with stand that heat, linseed oil treated tubing being one of them heat treated aluminum being another. ( and there are others)

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Like everything in life, there are trade offs.The Ura powders excel in some areas and fall short in others. They are by no means a cure all or direct replacement for liquids. For corrosion protection in extreme environments, its hard to beat a conversion coating, epoxy liquid prime and enamel ( stoved or air dry ) topcoat. And keep in mind if you are going to use a primer that has a sacrificial pigment, the sealing traits of powders can hinder the primers performance. So I go back to my basic point, use the powder as a topcoat, and prep/prime as you would with any other topcoat. If you just do a single stage on bare metal.....well good luck with that.


P.S. Just so you know, I use both powders and liquids, not that I am partial to one because I dont have the other or anything like that.

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Originally Posted by Aaron Novak

Like everything in life, there are trade offs.The Ura powders excel in some areas and fall short in others. They are by no means a cure all or direct replacement for liquids. For corrosion protection in extreme environments, its hard to beat a conversion coating, epoxy liquid prime and enamel ( stoved or air dry ) topcoat. And keep in mind if you are going to use a primer that has a sacrificial pigment, the sealing traits of powders can hinder the primers performance. So I go back to my basic point, use the powder as a topcoat, and prep/prime as you would with any other topcoat. If you just do a single stage on bare metal.....well good luck with that.


P.S. Just so you know, I use both powders and liquids, not that I am partial to one because I dont have the other or anything like that.

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Your beliefs are contrary to the findings of the Navy testing program that lead to the Navy changing to powder coating on their support equipment, the preventive maintenance program was having the equipment disassembled for cleaning and repainting ever 12 calendar months, when we changed to powder coating the period was extended to "as required" and the equipment was staying in service up to 5 years with out any other treatment.
Your conversion coating is by another name "Alodine" and others are an acid etch and there is much discussion in the industry as to it worth, because it will cause more corrosion when not removed prior to painting.
The iron/steel parts that we are using powder coating on will get an etch just like the aluminum then they are rinsed and pre-baked and then powder applied, and baked again. Thus the pre-treatment are almost identical and the paint film is much stronger using powder.

As I have said before, when done properly the powder systems are far superior to any of the wet systems.

Tom,
And your beliefs are contrary to what the material science department of the company I work for has learned about coating components for marine applications. This could go on and on.

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Originally Posted by Aaron Novak

Tom, This could go on and on.

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Yes it could,,,, Cya.