Be extremely carefull heating the bumper, it's best to heat the surface from the back, don't use the heater on the painted side ! Paint probibly will change color with the heat right at it a fraction to long, and you wil only know when it's to late. I would take a jack, a flat piece of wood on top(less conducting of heat) and raise the jack up until the bulge is under enough tension. Only then ! apply heat from the non painted side of the bumper and the bulge should even out automaticly when enough heat is supplied. In this way you can add just enough heat to smoothen it out because the bumper itself will tell you when it's hot enough, and you minimize the chance of overheating.