Freezing doesn't work, nor does compressed air. I did split the seam on an early CB750 tank and popped the dents out from the inside (the early 750 tanks seams were brazed, not roll-welded) on the blue K2 I restored a few years ago, but the guy I built it for had a problem with the tank sealer I'd used, so I ended up painting another tank for him.
The PDR system is an excellent method of removing a lot of dents from motorcycle tanks by trained techs who do it for a living. I love watching their vids on youtube, it really is amazing what they can do with the tools. Not just glue pulling, but special tools for pushing them out from inside the tank.
So, the best way to remove a dent is to bring in a PDR guy, he can tell you if it's repairable, and then fix it for you properly, for a fair price. Some things are best left to the experts.
