The best method to find a leak is by air pressure test. This is the same one used by the factory and all wet wing aircraft manufacturers including Boeing. For a Mooney the procedure is as follows:
1. Empty the tank completely and let it dry for at least 24 hrs.
2. Set the fuel selector to the opposite tank and install the fuel cap.
3. Connect a 3/8" hose to the tank vent and clamp it. For M20Js and before usea smaller hose and seal the coaxial vent tube.
4. Blow air by mouth only into the tank as much as possible. This will insure no excessive pressure will go into the tank.
5. Bend the hose or keep pressure into it (if it is a big leak).
6. On the suspected leak and adjacents areas spray SD-20 foamy cleaner and watch for bubbles. You may need to remove dry areas inspection panels to get to the leak outlet. You can also use a hose as an stethoscope to listen for leaks.
Boeing and other big manufacturers use ultrasonic generators in a pressurized tank and then listen for the sound wave coming out of the leak with an ultrasonic sensor. Same procedure is used for the pressurized cabin. For a 5,000 gallon tank no one is going to put fuel or water when air is simpler, quicker and cheaper.
