Some kind of tar on floor pan/chassis...

[EASY RIDER]

I have never tried this but I have heard that the inner tar board can be removed from the floor pan with dry ice. You might need a lot of dry ice but it may help in the hard to remove spots. You are supposed to put the dry ice on and let it sit. Cover it if you can. After it sits for a little bit you may be able to just chip it off. The cold temperature is supposed to make tar board and maybe your under coating brittle enough to easily remove.