OK, Mr. Fahrenheit351, per your request, here is what it takes to remove undercoating. I know there are several ways to do it, and after trying many methods, this is the best I've found. Make sure you are wearing really old clothes, shoes and a hat, because when you are done, you will want to burn them....
The process involves several steps. First, get out your heat gun, warm the undercoating and scrape with a sharp 3/4" wood chisel. The hardest areas are in the corners, and curves. Do the best you can, but there will still be lots left. Be extra careful on the back side of painted panels. If you heat them up too much, I'll bet you can bubble the paint. I haven't done that yet, and don't want to.
For the next step, fire up your 7hp compressor, and put a 2" cup wire brush on a die grinder. Wear a face shield and ear plugs, and go to town. You will make an incredible mess, and be covered with little black pieces. You can't do this step first, because the thick gooey undercoating will just clog the wire brush, and just smear stuff around.
Step three is disgusting. Take a pan of mineral spirits and a rag, and start wiping. Let the liquid soak in as you work your way around the fender. Don't let any area dry out - keep it damp. In five or ten minutes, go back and wipe off what you can. Mineral spirits will soften even the hard stuff, and will eventually get you down to bare metal. You can help it along with the wood chisel. It will take several application of mineral spirits. Each time, you will remove half of what's left. (If you can get down to one atom, then remove half of that, you have nuclear fission). By now, you have nasty brown goo all over your clothes, on you face, your arms and arm pits, and have tracked it all over the floor..
Final step is a rag and lacquer thinner. Plan on about ten hours per wheel well. -Steve
-Steve