when my tank rusted, it was on the overflow right beside the fuel pump. It was very hard to see as it was mainly on the underside of the tube. to fix it, i had to change out the entire tank, but as a temporary fix you can try mighty putty on a clean surface. Also, if you want an alternative that will work too is to go with aircraft gas tank sealer (resists constant immursion in 100LL, JET-A fuels... harsher than pump gas).