corrosion through powder coat

[ken nougaret]

I spent 5 yrs building my S7 and have been flying it regular for the last 2 yrs. I ordered my kit powder coated. I am seeing increasing corrosion on the horizontal struts, jury struts, and flaperon counter balance weights. I will do my annual soon and do my best to inspect the hard to see areas. My concern is in the areas i cant see, like under fabric ( maybe the fabric protects?), and the wing struts.
Have others had issues?
Ken

[Bud Davidson]

Ken, Very interesting. I am restoring a damaged kitfox from a kit originating in the mid 90's and found rust hidden by red powder coating. Not on fuselage tubing but as you note, on wing fittings. No corrosion inside wings.

[airlina]

Ken,Way back when I built my Series 5, (it was a 1999 kit) and Skystar was the kitfox manufacturer then, I found similar issues. The first section of my manual was builders tips & pg 1 , paragraph 1 says inspect all powdered coated parts for bare spots and touch up, however more than bare spots, I found evidence of poorly prepped bare metal pre powder coating that had light surface corrosion that caused bubbling of the powder coat. I recall taking all the powder coat off of some parts sand blasting and epoxy priming and repainting. Was not real happy with the lack of quality control back then as it showed up in other areas as my project progressed. Another big one was fuel tanks with manufacturing holes in the fiberglass that their customer service guy said couldn't happen because all tanks were pressure tested. You can imagine how the conversation went from there! Bruce N199CL

[ken nougaret]

Thanks Bruce. These areas that have a problem now showed no issues while building. They looked to be covered fine. This started showing up about a year ago

[ken nougaret]

Good to hear from you Bud!

[bbs428]

I thought only the older kits had these issues?

My 1998 outback kit certainly has them.
It was stored in a heated garage for 20 years. Had it's fair share of scratches and chips.
The powder coat on the bottom of the airframe was shedding like a bison in springtime.
I wire brush/chip away at the coating till I get into clean metal. Then it's "dip and dab" time.
acid brush some Corroseal on it and 2 part epoxy primer when dry.
http://corroseal.com/

On the smaller parts that are corroded, I sand blast them and paint as above.
I hope I get all of it before I cover.

I use Boeshield T9 spray for bare metal, connectors, battery terminals etc.
Leaves a nice waxy residue. I like it a lot. https://boeshield.com/

Other products work just as well but these are the ones I have used.
It's extra work, but that's what this bird is all about, right? Lol.

[PapuaPilot]

During my build I found corrosion under the powder coating on the lift strut tubes (see the picture). Thankfully it was just surface corrosion. I ended up stripping and bead blasting the parts. Then I painted them with Stits epoxy primer. I have a 1999 Model 5 and the parts were about 16 years since manufacturing at that time. The first owner lived by Salt Lake City and the Pacific Ocean in SoCal during the 14 years he had the plane, which may have been a big factor in the corrosion.

Powder coating is not the best way to coat an airframe, even if the preparation is done well. Of course any type of coating requires proper preparation.

[Dave S]

Powder coating is not the best way to coat an airframe, even if the preparation is done well. Of course any type of coating requires proper preparation.

Have to agree with Phil on this. Powder coating is sort of OK and it works for most, like anything else if done correctly. It looks nice. One of its foibles is the ability to hide stuff going on under the coating.


My local poly fiber guy, who has gone west, had an incredible amount of experience rebuilding tube and fabric airplanes (including float planes which had been dunked). He would do whatever a customer wanted for coating an airframe; but, favored polyfiber epoxy primer under aerothane. That has to be done correctly also, prep has to include complete degreasing, removal of any and all corrosion/rust (his favorite method was blasting - which also revlieves surface stresses in metals and gets all the welding slag off) making sure the surface is entirely clean with sufficient tooth on all surfaces to assure a very good grip of the surface coating. His painting method involved spraying each tube from nine directions; first the epoxy primer then applying the aerothane after the epoxy solvents were gone but before the epoxy was entirely cured - this resulted in the two becoming one very sturdy coating.


I chose to order a bare airframe - do the blasting, epoxy & areothane deal. Part of my reason was economics (I had the time, not the money) the other reason was to assure the welds were solid and everything was completely prepped before coating to my satisfaction. This was one of the last skystar airframes and I did find defective welds on the rudder pedal adjustment brackets. A person pretty much has to assume welds are good if powdercoated as it isn't possible to thoroughly inspect each weld with the coating on (although the same can probably be said for thick paint)

[PapuaPilot]

You said it much better than I could have Dave. Powder coating looks great, is easy to apply, but can hide things. I generally only use it for non-structural parts.

I am currently restoring a Wag-Aero Sport Trainer (i.e. J-3 Cub) and will be doing exactly what you said here with the Stits Poly epoxy primer. We are doing this plane as a project at our EAA chapter and are currently converting it to an L-4 Grasshopper.

[Flying farmer]

I fly a Model IV 1200lb Speedster in the UK. It first flew in 2001 and I purchased it in 2008. In 2013 I decided to investigate some small areas of surface corrosion on some lower fuselage tubes. I ended up removing all the fabric, blasting it back to bare metal before cleaning and spraying in 2 pack epoxy primer and finishing with Aero-Thane top coat before re-covering. Luckily I too discovered that it was only surface corrosion. I did find it concerning that the area of corrosion had spread under some areas of what otherwise looked like unaffected powder coating.
I flew it again in March 2015 and have had some amazing flights since then.
It is a great fun to fly and very responsive so was worth it in the end!

Michael Porter
Model IV 1200lb Speedster
Rotax 912ul