To be sure a fuel tank has no leaks it is best to test it with a solvent-type fluid colored with dye. This will work muck better than air pressure and soapy water. A fluid like gasoline or acetone or alcohol will wet the surface of the aluminum and find its way through small cracks and pinholes much smaller than air will penetrate, even under a pressure that would distort a fuel tank permanently.
So here is the recommended procedure:
Seal all tank fittings except the one you will use to pour in the indicator fluid. (I use the fuel filler cap hole).
Mix about a quart or so of either acetone or alcohol (stove fuel is fine) and Gentian Violet. (1 fl oz of 1% or 2% from the drug store) The reason for either of those two solvents is that they are both water soluble and therefore easy to rinse from the tank with water. I think acetone has a slightly higher vapor pressure, so I have been using at least a little bit of acetone in the indicator fluid I’ve been testing with.
After putting the indicator fluid in the tank, close all fittings, and roll the tank around so all the welds and fittings are covered in fluid for a few seconds. The vapor pressure of the solvent will pressurize the tank enough to help the fluid flow through any pinholes, cracks, or even punky welds.
After a few seconds (yes, just a couple seconds) any leaks will show up as if they were bleeding purple blood. Mark them with a sharpie, and then drain the fluid back into a jar or can. I like to let the test run for about 15 minutes or so to find even the smallest of holes.
When you are sure you have run the test long enough, remove the drain plug, and drain the fluid back into its container. (Be careful when you pull the plug, as even the small vapor pressure in the tank might want to blow the fluid into your eyes.)
After removing as much as fluid as possible, rinse the tank several times with water to remove any traces of explosive gasses prior to taking the tank back to the welding shop for repairs. A garden hose out on the lawn works great for this.
To remove the purple stains from your hands, clothes, etc., use a soap that has lye in it, like Lava. Works great! The same quart of fluid will allow you to test many tanks, and it will “keep” forever in a sealed container. I was amazed at the leaks that were found after none were found with the tank under water with 3 psi of air on it.