Looking for some ideas to remove rust patches from inside my header tubes. These headers are ceramic coated outside so I don't want to damage that coating.

I have same issue, I broke down and bought a stainless kit and collectors and having them fabricated. The only thing I could think would be any good is to protect the outside and sandblast inside as far as possible. There may be a better approach.

Huh? You guys have way too much time on your hands. LOL.

Have the inside coated....??

Take headers off and go buy a bristle brush for cleaning furnace and use that

Or small diameter chain in a drill