If you find your car is hard-starting, stumbling on idle or sluggish, your car may have a cracked gas tank for which you will need a gas tank sealant kit. KBS, Eastwood and AutoZone all make sealant kits. To locate a gas tank leak, you must clean the tank. Use a 20:1 mixture of muriatic acid and water or acetone to clean the gas tank. Once the gas tank is cleaned, place a piece of white cardboard beneath the tank to detect the location of the leak. Once located, drain the tank as far as possible and apply the sealant from the kit. Let it dry and refill the tank, watching for further leaks. If it is sealed, coat the area with good primer paint and sealer coat and you are finished.
For more info, see How to Repair a Gas Tank With a Hole In It