Wrench Tip: How To Remove a Bolt From Plastic
I had the recent displeasure of being rear-ended in the Vehicross. Luckily, the accident wasn't too bad (and we will get into it more in a later project installment EDIT: we did), but the bumper cover did need to be replaced. We decided to replace our damaged bumper with a used one. The used bumper was in good shape, but we found a number of the mounting bolts were rusted onto what was left of the bumper support. We would have to remove these bolts without damaging the new cover. This would require a delicate mix of power tools, cleverness, and a basic understanding of heat transfer.
The head looks okay. Rusty, but ok. This shouldn't be too bad, right?
Wrong. From the backside you can see the problem we have. This is actually part of the bumper beam and should not have come with the bumper (you non-salt states have no idea how lucky you are). The threads are completely fused to the weld nut. There is no way we are going to remove this with hand tools. 
Drilling the bolt would be difficult as the plastic is very flexible. The heat could also easily permeate the head and melt the plastic. A cutoff wheel seems like a good idea, but the act of cutting will also heat up the bolt and will melt the plastic. Fortunately, my dad is a clever fiend and has done this exact job before. So grinder (and brains) it is.
We were careful to cut only the weld nut and not the chunk of bumper itself. Cutting off the nut will free the rest of the metal and the head of the bolt will drop out from the other side. You may be noticing that this all looks a bit wet. Well the real trick…
…is to soak the bolt with a wet paper towel. Cut for 5 or so seconds, then immediately dampen the bolt. This will draw off the heat. It'll steam like crazy when you do, but this is good: the act of evaporating the water will draw away a lot of heat energy. If done right, the heat will never even reach the head of the bolt. While the backside here was hot enough to steam off water, the head was completely cool to the touch. 
Once you're about 90% through the bolt, use a cold chisel to knock off the last bit. This will prevent you from going through with the grinder and cutting the bumper (or whatever other plastic part you're working on). It may sound like this will take forever, but the cutoff wheel goes through the bolt like butter, so it only took about 5 minutes to cut, cool, and chisel the bolt off. 
Perfection! The bolt (on the left) came out quite nicely and there is no damage to the surrounding plastic (well nothing more than 100,000+ miles on a car already did to it). On the right is the result of the monkey fisted seller of this bumper. As annoying as this is, we did get a good deal on the bumper and these are exceedingly hard to find. We were able to save enough holes that the new bumper doesn't rattle at all and looks a hundred times better than the old smashed bumper did.
