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Corrosion
I have a corrosion issue which is a bit surprising in an aircraft with 470 hours that first flew in 2014. It is limited to a few specific places.
Worst area is the RH flaperon with patches of corrosion the full length, all located on the upper surface just behind the lap joint. LH is similar but not as extensive. Both outboard, rear upper surface wing skins have a patch also, again just behind the lap joint. There is also a small patch on the turtle deck near the front lap joint but not as close as on the other areas. There is no bulging of the joints. Doesn't seem to have extended beyond the areas that are obvious. Noticed the first patch a few months ago - the others have appeared fairly rapidly in recent times.. The aircraft has always been hangared and is well away from the coast. It was the builder's third RV, so expect he would have taken care of the kit during assembly. I have a paint shop lined up to fix it. Has anybody else experienced similar? I am wondering why it has appeared in these limited places (nothing anywhere else), and whether there might have been localised defects in the aluminium sheet. Jack |
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Is this on a painted airplane?
If so, what you are likely seeing is filliform corrosion. It is fairly commonly associated with skin edges and lap joints where moisture was trapped during the painting preparation process. It is why many years ago I stopped using water and liquid chemicals for surface prep before painting. It is very difficult to get all moisture dried from trapped areas. especially if the job is on a very tight schedule (like mine always are). |
Corrosion and Paint Prep
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Speaking of corrosion, I live near the ocean and get continued corrosion on the gear legs, I am told by Lockwood that this is very common in coastal Florida.
I had been scotch-briting, prepping with self-etching primer and painting. I recently order POR15 prep kit after talking with their tech folks. They claim better results. Anyone ever use POR15 on their planes? |
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It involves mechanically surface abrading the bare metal areas on the exterior of the airframe. On flush riveted skins this can be with a DA sander and fine sanding disks. The goal is to only 100% de-gloss the surface. Very little material is removed so the alclad layer is still intact. On thin skins (control surfaces) or areas with protruding rivets (RV-12) Scotbrite pads are used by hand. On larger areas with heavier / less fragile skins a round Scotchbrite pad on a hook-lock disc in the DA are used. Once the scuffing is completed, the parts are extensively cleaned with a surface prep cleaner and disposable lint free towels. The scuff to application of primer time must be kept to as short as possible because no conversion coating gets applied that will terminate the oxidation process. No more than 8 hrs if possible. The only corrosion instances we have ever had were on airplane painted after the traditional etch / alodyn process was done. We have never had a bit of corrosion or delamination problem using this process. |
Perfect timing Scott. I am heading to the paint supplier today to buy my paint and supplies. I was dragging my feet thinking about hosing my plane down to prep for paint. I like the idea of keeping parts dry. Any other tips to share?
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Just make sure it is scuffed real well. There should be zero hint of reflection in the surface. Clean until you no longer get any appreciable residue on the rag. Keep the time to primer as short as possible (no more than 8 hrs if possible) |
Certainly in the UK, we find that mainly QB aeroplanes or parts that have been QB'd suffer from tinworm out of the lap joints, mainly because there is no protection between the surfaces and as panting occurs with warm/cold outside/inside etc, condensation gets between the skins. Our 7 is still having issues.
Guys who do slow builds and scuff and prime the overlap joints don't have issues which is what I am doing now on all new builds. Overlap joints get wet assembled with a quick squirt of rattle can 1k epoxy primer just before riveting. |
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I agree Scott that the interior may have the wash primer, however its mating surface doesn't and I think that is where the problem starts when you get a moist environment. We regularly see QB parts tinworming after a year or so, but I also like your idea of an almost dry prep, we will be taking that forward for sure. We are having a lot of success with the wet join technique, not messy and stops any possibility of moisture getting under the paint film at a later date. I also advocate that once painted and cured, an internal spray with ACF50 or Corrosion X will be proper belt and braces.
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