The normal paint removers at the local hardware stores have been on the weak side for awhile now. Figure multiple applications + some physical scrubbing to go with it. What helps is if you scotchbrite / lightly sand the gloss off first other wise the stuff just sits there and kinda fizzles out.

Benco B17 is what powder coating places use to remove old coatings. There are YouTube vids that show it in action and it's nasty + potent stuff so I think it would go thru most paints pretty easily.