If the leak is in the top of the tank it may be possible to repair it in place temporarily and perhaps permanently.
I assume you believe it to be in the top because it begins to leak when the tank is full or near full.
Once had to deal with a very small fuel leak in the top of an AL tank.
First we accessed and cleaned the top of the tank. Have a good look for stains, wet spots and smell it up close looking for the leak.
Close to the leak it will smell like fuel and away from the leak it will have that slight funky bilge moldy smell. Check closely the hoses and clamps, (vent, filler and output) as well as the seal for the sender unit.
Put down paper towels and tape together to hold in place which should show up any small pinhole leak with gas stain especially if it is sloping out a little when you go up on plane and change the angle of the tank. Make sure the paper towel is under all the hose fittings and around sender. You want to paper towel map everything on the top.
Once found a small pin hole leak in the weld around the output tube welded into the top of an Al tank. The leak would only show itself when the tank was full. Procedure to repair was to get the fuel level down sufficiently that no fuel was leaking out. Then clean all surfaces around and in the small pinhole leak gently with manual tools. The hole will be a little bigger when you are finished. Power tools such as drills and Dremel tools with open sparky armatures were not used for obvious reasons. We used emery paper and a small drill bit and small cone shaped grinding stone fitted into a hand tool. Once the hole was cleaned out and any corrosion was removed down to bare metal, we then sealed the tank with a sealant specifically formulated to resist the solvent effect of fuel. The one we used is called “Seal All” and is available at Canadian Tire. Canadian Tire also sells other sealants for metal fuel tanks including some that will also resist methanol added to some fuels. They also have a kit that comes with sandpaper, a fuel resistant sealer, fiberglass webbing and epoxy etc. Read the instructions well and do your own research. This is not something you should just take on faith from some guy on a forum.
Found this to be a useful method for emergency repairs and have added this stuff to the boats emergency damage control kit.
You can also flush your bilge around the tank with hot fresh water and detergent and use fuel absorbent pads to clean up any residual fuel components/the residue left over when gas/fuel evaporates.
Better method is to drain and remove tank and take it to a professional and they will likely fill it with water and then weld up the leak but at that point given the work involved to drain, remove and replace, why not put in a new tank and avoid having to do it again, especially with an older tank.
