The dry ice method dates back. I met an old-timer, while I was stationed in California, who has been a body man since the 50's, and he told me that was a trick they used to pop out dents that were NOT creased. If the dent had a crease, you'd need a dolly and body hammer. He walked me through it, but it sounds like too much of a headache. I'd rather get a Ding King.
Frederic