FYI, automotive repair spot welders, are often capable of putting out 10,000 plus amps, have over 1000 lbs of compressive force at the weld point, and normally does a 6mm weld "nugget". The proper testing of a spot weld in the automotive industry is a "tear out" where the weld is torn apart. If the "nugget" stays in tact and the metals tear around it, it is a successful weld. These are often done on scrap panels that have been removed from the damaged area of the car. I don't know if that information is helpful to you or not. I just don't want somebody trying to put a car together using this method if it does not actually offer proper penetration. After training technicians on direct resistance spot welders, I see a few reasons why I wouldn't trust this method until I attempted a tear out.