Actually, there is a tool for undercoat removal. It is a "Crud Buster" or Dynabrade's "Dynazip". It's a wire wheel that really works well and actually leaves the metal with a light sand-blast look. Originally designed to remove ship barnacles.

Sometimes the dry ice works (liquid nitrogen always works but is a bit difficult to handle); heat guns, scrapers and torches work with a bit of danger attached.