Surely you mean he just coasted the inside of the bed with undercoating????????? Try some ACETONE to see if it will work any better. The undercoating should be more pliable than what the Line X is so you may be able to scrape some of it off with a plastic spatula or try a razor paint scraper. You will remove some of the paint but cleaning it all down and reprime before putting the Line X on. You may not need the primer but ask Line X if it needs to be primed.
I've actually had pretty good experiences with a wire wheel on a grinder. you're likely to get it a bit too far in some places and need to touchup the base primer but otherwise that will leave a great base for the linex to adhere to