Can I use this transformer for a DIY spot welder?
For spot welding you need high current at low voltage, so you will use the transformer to decrease voltage. You should completely remove the primary (high voltage side) winding and replace it with just a few (maybe two) turns of thick cable for the welder and input power from the battery to the formerly-low-voltage side (now it is higher) which should have thick enough wires. So you won't use those thin high-voltage-side-wires anyway.