Question- Powder Coating of aircraft parts

[nomocom]

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.

[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

[Tom Downey]

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?

[nomocom]

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

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......

[nomocom]

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

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.

[Tom Downey]

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.

It's just a nomer for any ____urethane product.

[WLIU]

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

[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.

[Tom Downey]

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.

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.

[WLIU]

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