I was able to put my rotisserie to some good use today and was able to start peeling off the layers and layers of tar from the underside of the car. I don't know what this stuff is, but it seems to have saved the vast majority of the underside of the car from rust, but not dents.
Last summer I was able to get the vast majority of the tar off, but I was surprised to find how much more remained. So today I scaled off the rest the best I could, and used my grinder to start taking off the layers to get to the original paint and primer. I use some "gunk" remover spray to loosen things up, my heat gun and a trowel to get majority of the tar off before using the grinder. I ultimately was able to reach nice clean bare metal. I figure that if I could do a bunch of this legwork out front the cost of blasting it will go down, though I've yet to speak with the vendor. I hope to do so later this week.
Here are a few before and after pictures. And I was really only able to get to one large underside panel.