I used a product sold at NAPA stores, specifically for gas tank repair. Small kit, mix two parts together, quickly, and quickly spread on the gap/leak. I hardens qucikly and is VERY HARD within a few minutes. So far it is the best/only thing I have found that seals.
I used some JB weld the year before and it began seeping after about 3 tankfuls, slight, but still a leak. So far the NAPA stuff is holding well. On the 10th or better tankful, 1000+ miles.
When mixed it can be spread, kinda like drywall mud, and it is a dull grey color, so against a bare tank it would look close to the same color.
It costs about $14.00 for a small package.
Bob