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

[Bullyboy]

I found it works best if you heat the scraper, not the tar.
Then it comes off in bigger sections because you are slicing it off.
If you heat the tar it just turns into mush and globs up.
This works good to get tarboards off in one piece if you have a long scraper.
Then you just have to clean up with laquer thinner, acetone, varsol, etc.