Patch with POR15 paste after priming with POR15. Must be completely fuel free but need not be rustfree only loose rust and flakes. You can use fiberglass with POR15 also. Use their cleaning products for complete success..
I use RTV orange silicon to repair the fuel tanks on my diesel trucks. Clean well with brake clean,. then spread an inch or 2 past the leak on all sides. Generally 1/4 inch thick and give it couple days to dry before filling with fuel. Lasts couple years.
As someone who had one of their tanks fail last Spring while on the hard and the bilge pumps kicked on resulting in a $20,000 clean up bill, I would reccomend you keep a very close eye on the situation.
I know it sucks, but proactive tank replacement is probably a lot cheaper than a clean up bill, and would give you piece of mind. Just my 2 cents.
i would be surprised if a coating you pour into the tank would fix it for long. a steel tank can rust inside if there was water in it. if you can get the tank out a new piece of steel could be welded in. ford tried coating the inside of the tanks on there diesel trucks in the 2000,s the coating would come off in very small pieces. in the 70,s i worked for a mopar dealer and they had trouble with there gas tanks. the fix until we could get a new tank was to use soap. yes bar soap from a&p. take a knife and cut some from the center where it was soft and put that on. sometimes it would last for months
Could you put a bladder inside the tank? If any of our steel tanks ever leaked it would be my first thoughts. I do note that last year on another boat we used a MBX bristle blaster to give an oil plan (off the motor) a sandblast type profile and glass-epoxied the exterior of the pan which had corrosion in multiple areas. We figured this would be better than just putting another thin oil pan on. One time we did treat a rusty bulldozer tank with the pour in epoxy too, but we thoroughly cleaned the tank first. Drying a tank I figured out worked best, take some of the old style Christmas lights, push them into the tank, really heats it up nice to dry it out. The wife's hair dryer didn't last 10 minutes!