When I ran a high school body shop we used that procedure pretty effectively. Once the ball is inflated a little, tapping around the perimeter of the dent with a flat hammer helps eliminate creases. Some of the videos I've watched concerning the "paintless" process employ the same technique, using a rawhide mallet or a rubber hammer. A text on metal bumping I taught from frequently was written in the mid-1930's, and included lots of techniques for metal finishing that minimized the need to use lead for final finishing. Bondo was still a couple of decades in the future when that book was written.
Jerry
