Most metal fuel and water tanks occasionally develop small leaks. These usually occur along welded seams and can come from faulty welds, electrolysis, excess flexing of the tank, or acids in diesel fuel.

Most leaks are pin-size, tiny holes, and can be closed with a punch and hammer. Once you find the leak point, take a center punch and/or a "drift" (like a center punch but without the point) and "punch" the leak point. This will smash the soft weld metal into the hole to for a seal (this is the process that builders use to seal small leaks when testing tanks).

The difficulty is in finding the leaks. To find them use a very low air pressure, not more than 1.5 pounds per square foot, to pressurize the tanks. Then take soapy water and a small brush, and go over the areas where you think the leak is until you find the bubbles which the air will make in the soap. When you find the bubbles, look very closely with a good light and sometimes a magnifying glass to find the exact leak point. Then "punch" it to seal the metal. Welding is rarely required for these situations.

To pressurize the tank, you will need to close the valves leading to the fuel system and water pump. Then take the air vent and put the air pressure into that point. Use a large-diameter, high-quality pressure gauge, so you know you are not putting in too much pressure. If you over-pressurize, it is possible to bend the tank tops, break welds, and do all sorts of very expensive damage!!!