I'd recommend adding a resistor across the capacitor, to drain the charge away when the welder is turned off. Otherwise the capacitor will remain charged, which could cause an unpleasant surprise for the unwary. Voltage on the capacitor will decay at a rate determined by the time constant, R*C. For a 50,000uf capacitor and a 1-second time constant you will need a resistance of 1/.05 (50,000uf = .05 farad), or 20 ohms. It will have to have a high enough wattage rating so it doesn't burn up. I went with a 50 ohm resistor when I modded my HF welder for DC -- high power 50 ohm resistors are fairly common. Stores selling stuff to ham radio enthusiasts would be a good place to look.