Introducing heat to the panel will only make matters worse. The heat generated by the welding process, caused the metal to shrink within the blued area (Heat Affected Zone or, 'HAZ') In order to reverse the effects of the welding process, you must return the metal to it's relaxed state by stretching the metal along this HAZ. Never work the metal outside this area when dealing with a weld seam, or you will have a huge problem to contend with. If you have an obstruction, rather than using a dolly, you have to be resourceful and use a slapper or other thin, hardened piece of metal to resist the blows from the hammer or slapper you are using on the face side. I prefer to always use a slapper, as the contact area is larger, yielding much smoother and controllable results.