The "chalk" is the rotted paint. I'm no expert by any means. But I have tried to buff rotted enamel. It is just a waste of time. You can seal it with some success and then repaint, even rattle can will work better, and probably be easier in the long run. I have been told by experts, that once it is rotting, the only sure way to restore it is to get it to bare metal, and repaint it.