Here are some mufflers I did.... using rustall... But I have and work around rusty equipment that gets hot.... I find that it does not flake and you can see bare metal through the rust. This is as long as it is in continuous use of course... if it sits.. it rusts... but once heated again on a regular basis the flakes drop off. What I did is after the rustall dried I gently rubbed my fingers over it to knock off the flakes. Of course I have NOT added the effects of the exhaust yet.

KT