I have used JB weld for many things and it has worked great. The biggest thing that I have used it for is for plugging two holes in a car rad. They were the holes for the trans cooler as I put an external cooler on. they where the size of a dime and I used dimes to plug the holes first. I just traded the car in after putting 70 000 kms on it and the JB weld never moved or leaked. If you can get your hands on this stuff it would be your best bet.
The nice thing with it is you can put it on, let it dry then sand it smooth, prime it and paint right over top it. As the fuel tank isn't pressurized you shouldn't have any problems with it leaking. I would recommend that you take the tank off and remove all the gas, letting it dry out for a few days.
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