Mitch, I'm going to add some of your PM....
I have a shrinking hammer (serrated face) but I'm not sure if it's the correct tool. I've tried it in a few spots, both with and without a dolly on the backside, but it just seems like I'm creating a knurled surface.
Two primary shrinking hammers, the rotating type carried by Eastwood, and the knurled you mentioned (what I refer to as the waffle face).
Rotating hammer: I have used this one before, and have found that in order to get the face to grip and twist (shrink) the metal you need to use quite a bit of swinging force. The downside is that there is a fine line between shrinking just the right amount and now you stretched it from hitting too hard. My experience has been that any hammer can shrink, and light taps using regular body hammers may give more controlled results than something that requires a harder hit to provide the rotational force. Counterproductive, to a certain extent. I consider this a gimmick hammer that is difficult to control the results. Perhaps it was the operator in my instance, and someone else may have better luck. If so, good job!
Waffle face hammer: This one has been around for quite a long time and produced by some quality hammer makers, so it must work! It also has been labeled as a gimmick hammer, but that may be due to misuse.. To explain, lets start with Carpentry as an example. Estwing makes framing/claw hammers in both a smooth face and the waffle face pattern. For some that may not be as skilled in connecting the hammer face to the head of the nail, you may see a glancing blow that tends to bend over the nail. Replacing the smooth face hammer with the waffle face one shows less issue with nails deflecting as the waffle face can really be considered as multiple hammer faces. One part of the hammer misses and another picks up the slack. So as it relates to body work, the waffle face is strictly intended to provide a multitude of "hammer faces" to hit and not slip. I think where the misuse of this hammer comes into play is that most of us have swung this hammer like it was the Estwing framing hammer, which is not the intent. This hammer should ONLY be used with light taps, and if you are leaving a knurled surface, you are hitting too hard. Again, it is designed to hit and not slip using light taps. In most cases I don't recommend either of these two hammers to novices as they are so persnickety to get them to work correctly. I find the waffle face to be less of a gimmick hammer, so if you must choose one, use the waffle pattern hammer. If you feel you can't do without the Eastwood rotating hammer, look on craigslist for a used one.. Nobody uses them long after buying them, so you may as well learn your lesson at half price..