Has anyone ever refinished a hardwood floor?

[CheerfulYank]

We're doing ours. It had a layer of fake vinyl linoleum on top that was supposed to look like stone. We got that off. Then a layer of plywood. We got that off. Then some sort of linoleum-y layer that- is probably full of asbestos. We pulled that off but there's a black tarry residue left. How to get rid of it? DH says we can't sand it off as it would gum up the sander. We've just been scraping at it and using some sort of solvent stuff, but it's not working very well.

Any suggestions?