How do I get a cat urine stain out of a fiberglass tub?

We keep our cat litter box in the tub so it's out of the way - don't use the tub, use the shower. Anyway, I kept smelling a cat urine smell and couldn't figure out why as we clean the box twice a day. Lifted the litter box and there was a urine puddle underneath from one of the darlings peeing in the tub beside the box. No problem cleaning it up and getting the odor controlled, but now there is a urine stain in the fiberglass that even resists straight bleach left to sit. Any ideas??