I don't think you are going to have to worry about whether to blast that undercoating...blasting won't touch it. It is basically a sound deadener. If it is solid, you might leave it. So often water gets behind it and it rusts and falls off in chunks. It will take heat (carefully) and scraping to get it out. It is OK in there unless you ever have to weld where that stuff is located. Not a pretty sight.
