Yes, definitely derust the tank. If no more pin holes occur, the POR15 urethane kit could possibly cure that with just some painters tape and wax paper over the hole on the outside. I'd probably let a thicker layer cure there or possibly buy two small cans of the urethane for a double layer.
Let's talk a lot more here if you chose the POR15 route for this repair. Details, details, details!!!
Rick
Sent from my SAMSUNG-SM-N900A using Tapatalk