Butt welding of sheet metal is based on the resistance spot welding process. The difference is in the contacting of the materials.
In spot welding, one spot welding electrode is contacted from the top and one from the bottom, in butt welding only one spot welding electrode is used from the top. A surface electrode is used from underneath or the lower material (e.g. chassis) is contacted by means of a clamp or screw connection. This process is used in body construction and body repair.