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.Originally Posted by Tom Downey
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.
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.
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.
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.
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