If you're interested, this video is plastic welding in a nutshell lol. YouTube - Plastic Welding with Soldering Iron A friend of mine took an auto body program during high school and learned the technique there.. it's pretty simple and saves a ton of money, its just stinky. It's similar to metal welding, just using lower temperatures. You can use a soldering iron and a chunk of plastic similar to what the bumper is made of (a junk bumper can be cut up to use). If fixing a crack (like I'll be doing, similar to a butt weld), file the edges to 45º angles where they will meet like a V. Melt the plastic into the V and smooth it with the soldering iron to ensure the filler melts the 2 pieces together. To reshape a plastic bumper you really just need a heat gun. Sometimes you can fix deformations by stuffing a rag behind the dent and pushing it out on a hot day.