All you do is fill them up with water, let them over fill, that's it. You have it on a table, and if you're really worried, take a match, put it on the end of a long stick, and stick it in the hole. If you filled it up that way I described, you will realize how unnecessary that step is.. That takes out other fluids. I have fixed many gas tanks. If I had that tank, I would say it would be about a 3 hour repair, not including painting, but that tank would be solid. I've welded a lot of those round gas tanks you see of dump trucks. The trick is to cut off and end so when you weld it, you got every spot inside and you can fill the tank up with water before you weld the cap back on and see if it leaks. Motorcycle tanks are easier because the welded seam on bottom is really easy to cut and follow the seam on the inside. If you own a welder, learn how to use it. IMHO