Martinsr speaks well on this matter. My favorite option in bolts in sheet metal is to put a good wrench on the nut and the bolt, then twist, break and replace. Where that is not a good option, I find repeated applications of heat/cold water or heat and apply pariffin, which will seep into the threads. Also, the use of a good penetrant such as PBBlaster, Kroil, or CRC will help. Impact wrenches are a big help, too.
