I purchased a spot welder a year ago to manufacture camp stoves. I found that manufacturing the stoves was too costly, but I did manage to get a nice usable prototype out of the project.
The spot welder is normally too powerful to weld thin sheet steel, so I used an autotransformer (variac) to lower the supply voltage to the spot welder. Using this arrangement, the voltage can be set for any gauge steel spot
welding without burning through the steel. I do most of my sheet metal
welding with the voltage set for 95VAC. The variables associated with resistance
welding are current, weld time, and pressure.
Here is the prototype alcohol camp stove. You can see some of the spot welds. The stove still works fine after a year of use.