Black tar-like substance on a 77 Corvette body

Hey guys.

My Dad recently bought a 77 Corvette, lets just say the guy who he bought it from is one of those guys who should never own a car...

(the interior was painted with latex house paint and a nylon brush, for example)

...We did a lot of work on it (interior and engine) but when it came to paint, something disturbing arose. When the body was sanded down, there were large patches of a black tar-like substance on spots on the bare metal. As he tried to sand it off it just spread around. after a few hours of messing with it, they just decided to primer and paint over it.
Now the paint is bubbled and peeling all around where this black junk was. He is willing to have it stripped down and painted again, but he does't know what to do about the black stuff.
Also another option he is looking at is just sanding and repainting the damaged areas, but again, not sure if the black stuff in gonna mess it up again... or maybe it was just a coincidence in the first place...

thanks for your help

Bare metal is an interesting clue.....if you sanded to metal, then you sanded clear to the inner structure......all Corvette bodies are SMC in that era. Are you sure you hit metal??????

Quite possbily not, my Dad just explained it as under the paint and primer, I just made a dumb assumption

Either way, there is still a black substance that the primer won't stick to... and its ruining the paintjob... Help please.