Posted some pics on my project thread. I'm very happy with the results. As promised, some tips on lessons learned:
- Vet your vendor. Look for an owner-operator who does his own work. Stay away from high volume shops with large crews, and consumer shops focused on rad colors for the cool dudz.
- You don't need an "aviation" shop, but you do need someone who will pay attention to your specifications and ensure they are followed.
- Oven size is a potential issue. Measure your frame. I figure 20' long is the minimum.
- Get the process specs from ACA. Phosphate wash is critical, per Dale.
- Shops tend to like one powder brand. Each major brand has similar product lines. You don't have to use the same product codes as the ACA specs, but you do need equivalent performance. You'll need to do some research to cross match.
- Get instructions on ACA on which areas to mask and plug. These instructions may be vague. As a general rule, mask or plug anything likely to be a tight fit during assembly, such as the h-stab studs, landing gear bolt holes, and any bushings. Zip ties are a good way to identify holes to plug.
Now to the question I raised at the beginning of this thread: is powdercoating suitable and worth the cost for aircraft frames? I did a lot of research, and everything I found pointed to the same answer: hell yes. That was confirmed for me when I got my frame back. This stuff is insanely tough.
But where did the rumors about powdercoat not being effective at corrosion control come from? Well, that takes a bit of detail to address.
Basically, just like there are a lot of types of paint with different properties, there are a lot of types of powdercoat with different properties. Broadly speaking, there are 3 chemical categories of resins used: polyester, epoxy, and urethane. There are also 2 types of processes: thermoplastic, and thermoset. Thermoplastics melt, while thermosets react chemically to heat.
The stuff most people are used to seeing is polyester thermoplastic. This is basically plastic goo that melts and hardens. It goes on very thick, which makes it very durable and results in smooth surfaces that hides things like weld fillets. That makes it a good choice for things like playground equipment, sports gear, lawn furniture, etc. But, it does not chemically bond with the metal, and can be peeled off or even melted off and reused. That means the only corrosion protection is the unbroken barrier. Once compromised, it will not protect the underlying metal.
Polyester is also not UV stable, and not chemically resistant. MEK will strip it right off, and a few years in direct sunlight will fade it.
The great majority of consumer powder coating is done with polyester, because it is cheap, very easy to use, and comes in many very bright colors. If you go to a powdercoater and don't know what to ask for, that is probably what you are going to get.
However, there are several classes of industrial thermoset powdercoats such as epoxies, urethanes, and blends which chemically bond with the metal like paint. Epoxies are thick, highly corrosion protective and chemical resistant, but not UV stable. Urethanes are thinner, but highly resistant to UV, chemicals, and corrosion protective. Various blends exist to combine the properties of polyester, epoxy, and urethane. These types of powdercoats are used widely in mining, architecture, aerospace, and defense. There are also zinc rich powder primers used for marine environments.
ACA's go-to choice is Axalta Black Magic. It is a urethane thermoset in matte black. That must be a pretty popular option, as every major brand has one.
So how did powdercoat get a bad reputation? IMO somebody got a bicycle done in cheap polyester, it got a few nicks and rusted, and the internet stories started. But IMO that is as logical as painting a boat trailer in latex interior paint, dunking it in the ocean, and claiming the ensuing rust as proof that paint does not protect metal.
Bottom line, most of the major aircraft manufacturers powdercoat their steel components now. ACA, Vans, Carbon Cub, Maule, Husky, all do it as standard practice.
