Many bumpers on modern vehicles are made from plastic, not chrome, as they commonly were in the past. When heat is applied to certain materials, such as plastic, they become pliable. This characteristic makes repairing a plastic bumper easier because of its ability to be reformed. Heating a bumper prepares it for a repair and reduces the risk of cracking it. Heat is not normally applied to chrome bumpers. Most chrome bumpers are sent to a professional to be rechromed or replaced.
Inspect the damage on the bumper. Looking for cracks and dents will determine what type of repair you need to make and what type of heat you should use. Cracks and dents require different types of heat.
Direct the heat from the heat lamp to point at the bumper so the bumper can warm (automotive heat lamps are found at auto body suppliers). The heat lamp will warm the bumper and help to maintain the temperature of the work area. If the area cools, the plastic become less flexible and difficult to repair.
Use a heat gun on the damaged area; heat guns can be bought at auto body suppliers or hardware stores. Turn on the heat gun, and run it back and forth across the damaged area until it becomes flexible. Do not make the area so hot it becomes untouchable.
Reach behind the bumper, and attempt to push out the dent from the back. Some dents are more easily removed than others. Not all dents can be removed this way, but you can use this technique to get most of the dent removed before you finish the rest of the repair.
Turn on your plastic welder to warm it. Use your welder to repair any cracks in the damaged bumper. Your plastic welding kit should come with plastic rods to repair the crack. These seal any cracks in the bumper.
Use the flat part of the iron to press against the plastic rod and bumper. Take your time running the iron across the cracked area. The heat from the welder will become hot enough to melt the plastic rods and bumper together to seam in the crack and make it whole.
Old plastic bumpers might be too brittle to repair. You might find an old bumper cracks easily and is less susceptible to flexing.
Heat lamps and heat guns give off a high amount of heat that can be dangerous if you do not protect yourself. Wear gloves to protect your hands from high temperatures and glasses to shield your eyes from debris. Shop coveralls can protect your body from any plastic that might melt and drop on you.