A lot of people on the list have addressed the various adhesives but that isn't where your problem is. Rather the problem is that as you pull the dent, the edges delaminate becauuse they are up to whree they belong and thus stop moving. This allows air in and thus the problem. You need to have a flexible puller so that it will flex with the metal. I'd probalby use some rubber to attach to the metal with a good adhesive like weatherseal adhesive (high tack and strength) and then a relatively thin metal puller plate over that. IN addition, you may also want to increase the pressure on the metal from the backside with another rubber coated surface
-- Why do penguins walk so far to get to their nesting grounds?