Plug in the blow dryer and direct a blast of hot air on top of the ding for 60 seconds. Use the highest setting if your blow dryer has multiple heat settings.
Turn off the blow dryer and set it aside. Shake a can of CO2 air duster for computers and keyboards.
Turn the can upside down and aim the long straw nozzle directly at the dent. Press the button and blast the dent with CO2 for 10 seconds.
Watch the door as the hot area and the cold ding react. The dented part of the door will pop back into shape with an audible "pop."