I suppose it's possible to butch-up a repair of a leaking fuel tank by slobbering the interior with some kind of goo, or an epoxy of some sort on the exterior, or other band aide measures. They may well work for the remaining time that particular owner owns the boat. Or not-your mileage may vary.
Enthusiastic references have been made on this forum to tank coatings used in the aviation industry. No mention of the FAA-mandated routine inspection of such coated tanks, FAA-mandated removal and replacements of such coatings to retain aircraft certifications, no mention of the cost associated with the apparent success of such coatings in aviation use, etc.
I've also read on this forum an advocate of opening his tanks, crawling inside, and having a welder repair leak points from the inside. Seriously? I made a similar inquiry, for the same reasons, a couple of years ago in the Everett, WA area. I was met with absolute refusal by EVERY welder I could find to weld inside a tank previously filled with ANY hydrocarbon (ie-diesel), inside a flammable boat. Can you spell explosion, asphyxiation, or fire?They thought I was crazy, and no one would risk it. I'd love to meet that guy's welder. I expect he is a few bricks short of a full load.
Should you be fortunate enough to access all areas of either actual or suspected leaks, via inspection ports or man-sized access ports, then gooping them up may well work for you. It may well work for many years. But do the next owner a favor and disclose this fix to the potential next owner and his surveyor prior to sale. I'm betting they run for the hills. I certainly would.
All rants above are my personal opinion, of course. They are, however, backed with solid, direct experience in diagnosing and repairing leaks in water and fuel tanks aboard pleasure boats, and other professional experience in related subjects. So, for what it's worth, my conclusions for all leaking fuel AND water tanks aboard pleasure boats is remove and replace. Sounds harsh ($$), and it is. My safety and piece of mind is worth the money, and my conscience is clear when I pass on my boats to others.
Regards,
Pete