Heat gun and a putty knife! Then use wax and grease remover.
It will work..................................................................................................................................................................................................................................................................................................eventually.
I can just see using the needle scalier (great on rust!) and a heat gun on the OP's "thick tar" problem now.
Maybe it could turn into some kind of new rat rod look...
A Car Tattoo.
Seriously though,
W&G remover doesn't work that well on tar.
Your better off if you use reducer to cut the tar and then clean up with W&G as normal.
And by 'better off' I mean one must be
$pretty well off$ as well, to have enough money to use reducer (or the chem of your choice) to get an 1/8" of tar off the bottom of a cab.
And lots and lots of time.
Cold and a chipper of some kind (like 52Carl mentioned would work great if you had one) was the quickest/ easiest route I've found to remove the "old thick tar" undercoating I've come across.
Also as a word of caution: I strongly advise NOT to use a heat gun after applying Reducer to tar unless adequate
Destructive Intent is being applied...
