Heard of an interesting approach to removing undecoating, and even some paint, the other day while having lunch with some other car buddies. Some of which prep production cars into rallye cars.
Liquid Nitrogen. Freeze it, tap it with a hammer and watch it fall off. The hardest part is getting the set up to spray the liquid nitrogen. Your local welding/gas supply should be able to hook you up and give you a safety run down.
The guy telling me about related a story of three guys completly stripping an E36 BMW coupe to bare metal, inside and out, in one eight hour day.