This subject comes up every year or so on the list.  Based on my very poor memory the product you need is manufactured by PPG Airospace and is designed specificaly for repairing fuel tanks in the aircraft  and homebuilt experimental industry.Several years ago I saw a tank repaired with this product and until acouple of years ago when I lost track of the boat the repair was still holding. I don't remember the exact name of the product but it was extremely sticky and after curing was suppose to stay flexible.Most of the reports that have come back to this group over the years suggest that if the top of the tank rusting out is not the problem then the leak will very likely be in the bottom of the tank where water has lived. The tank that I saw repaired was drained slowly over a week or more to see where the leak stopped and it didn't proving that it was in the bottom.  A hole for an inspection plate was cut high in the tank to allow inspection and the black iron material used to build the tank was found to be very sound but the welds from the original fabrication looked questionable.The boat owner then called PPG and asked to speak to an engineer.  This man assured the owner that they had been manufacturing these products for many years and took the time to explain their various products, recommended the one he felt should be used and explained how to prep the tank and use the product.  The owner sealed all of the welded corners in the bottom of the tank and a few inches up the side's, waited the recommended curing time and then started to slowly add fuel to the tank.  The leak was sealed and was still holding several years later when the boat was sold.I would suggest that if you want to tackle this job or maybe hire it done that you call and talk to a PPG aerospace engineer as much of this product information came to me second hand.Good luck! John SiscoeM/V Chatea DeuxCanal Lateral a la LoireLere France Sent from my T-Mobile 4G LTE Device