They're probably cast zinc, and low quality zinc can be subject to "zinc rot". Another reason zinc gears don't make good gearboxes. I saw a lot of it on the detail items on my mato all metal P3. The zinc just seems to get all crumbly, if that makes any sense. I can't say for sure that's your problem, but it would be my first suspect. I would put them on a tank that doesn't get run much, or maybe even a static tank. On the other hand, if you can get video it might be fun to run them to failure. I made a video like that when the zinc gearboxes in my Pershing went bad and it came out pretty good. Actually watching a gearbox fail isn't something you see every day, and I don't think I've ever seen video of zinc tracks giving up the ghost.

Anyway, to answer your question, I believe the problem is in the metal itself and paint had nothing to do with it. Hope that helps.