All the ideas mentioned are proven, nobody has mentioned oven cleaner yet.
The stuff is toxic and nasty but usually gets mentioned when undercoating removal is brought up. It works, but you do not want that junk dripping on your tender parts. ( We are made primarily of tender parts when it comes to things intended to dissolve charred organics. )
I used it up in my rear wheelwells and the stuff wiped out like melted butter compared to smearing, melting and genereally gumming up with wire wheel efforts.
