Jim; you are correct; that is the crash I was (wrongly) refering to...Originally Posted by Jim Hann
My apologies to all the powder-coaters I might have impuned.
Chris
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
You Tube only proves that more airplanes have crashed due to Video Camaras than any other single reason...
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
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?
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
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.
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?
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.....
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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