After pumping out the WVO, draining some of the coolant, and unplugging it, it had to be very thoroughly cleaned, as aluminum welding will not work with impurities. The lid that accesses the copper coil and fuel intake will need to come off as well. So the task is to grind it down at the seams, both with a grinder and then with a wire brush to get the oxidized layer off. Then clean it inside as well (try a radiator shop for that part). Even then you can count on some residual in the cracks. So once it was finally welded, both seams at the bottom to be save, then you want to go over it and find the pin holes. The test is to put a gallon of paint thinner into the tank and feel with paper towel at the seams of both sides, to pinpoint any further pinhole leaks. In this case there were no more leaks. Hook it all back up, silicone the lid back on and good to go.
The image below shows what the copper coil looks like after almost 30K miles or 3/4 years of driving. Ideally you leave the oil level above this coil, so that you don't get corrosion or algae growing on it.
