removing undercoating.

I have tried all of the suggestions you guys have tried. Have to agree with Raaf69 it just a sanding disc forth grinder and a lot of time grinding it off. Not sure if I was doing something wrong but made a hell of a mess until I gave up. Started grinding and in a few nights I was done with it. But glad I did this since there was a lot of rust under the coating. Like some said rusting from the inside out or not properly prepped when first done.