I used a putty knife and then a braided wire bit on my drill. The stuff just flew off. Wear safety goggles and a long sleeve shirt buttoned all the way up, gloves and a hat. That old gunk stinks to high heaven and will make your skin stink for weeks....yes..even after you wash...
After I did all that, I still towed the truck on a dolly to the steam cleaner. It was easier to clean with steam and water after that layer of crud was gone.
My $.02..
