If it's an old car the ethanol that they now put in the petrol may have perished the fuel pipe (it's happened on a car of mine)
You can weld tanks if it's a hole but if it's corroded it'll probably be goosed anyway. As an aside I know that guys welding motorbike tanks always used to play the torch into the filler cap (after thorough washing out) on the understanding that if it's going to go bang it's better to be expecting it rather than while you are hunched over it, apparently petrol tanks fly through the air rather well!
I bet it'll be a pipe, gasket, or connection somewhere.
Good luck!
