I use Caswell products, available here in Canada. They are used for complete tank lining. Years ago my K7 had a tiny pin hole, in one rear corner. The bottom chrome trim strip had allowed water to collect there for many years.... the tank was so spotless inside, I did not want to line it and the paint finish was excellent, so didn’t want to damage.
I cleaned out the tank, as per the Caswell instructions. Mixed up a small portion of the liner chemicals and poured it down a funnel and tube to that corner. Keeping that corner low, swilled it around a bit and then stood it on end to dry. Basically a “local patch”, not visible from the filler neck. It’s still holding. Might be worth a try?