A two part epoxy like Liquid Steel will seal a porous weld if it is applied properly in a non pressurized tank. The most important factor in application is cleanliness. If there is any fuel or dirt on the surface of the metal it will no work, so if the tank is full and still leaking it gets to be very difficult to clean.
If the leak is in a threaded fitting, it is best to remove the fitting and apply a Teflon thread sealant and reinstall to correct the leak. If you try to correct a leak in a threaded fitting with epoxy, it is very difficult because of the need to clean the fitting all of the way around it and apply the epoxy all of the way around, and it is very difficult to adequately clean the threads.
That being said, look very carefully at the fuel filler hose as at least 95% of fuel tank leaks are these hoses.
Gerald