If you are looking to paint the tank, you can just paint the outside. If you are looking at painting things inside the tank, consider epoxy paints. I have painted several fiberglass tanks with epoxy paints and will be painting a very large custom built wood sump soon with it. With the epoxy paints we let them cure for about a week, then soak them in ammonia and give a good scrub, rinse and then use.
Dan