That 100% depends on what's wrong with it. Presumably it leaks, but from where and what's the condition of the rest of the tank. Gas tank repair putty can be great, but not if the entire lip is rusted.
step #1 find the hole #2 heat area with some sort of flame (ie, torch, lighter, etc) #3 if you find yourself in the burn unit... you did something wrong.... repeat steps 1 threw 2
mine leaks too, but only if I overfill it, so I guess its leaking on the top. I'd like to repair it but I plan on ripping the whole car apart and rebuilding as a hobby... so maybe I'll go new.