Tar-like undercoating on 63 Valiant...any way to remove it?

Goo Gone working well, but areas where the undercoat is thicker (corners, and nooks and crannies) are more difficult. Second coat of GG left on till next evening worked even better! May try scaler on the frame, especially those thicker spots, but I'm discovering rust spots in the floors, so probably won't use the scaler on those.

I'll be doing the primer/spray bomb on my back, as well...if I took it down enough for anything else, I'd never get to drive it!