The best type of paint to use to paint a fiberglass shower is an epoxy-based paint. Painting a fiberglass shower enclosure should be considered a temporary solution as the paint begins to peel after a couple years.
Prep work is the key to good results when painting a fiberglass shower. Sanding the shower surface with a 320-grit sandpaper roughs the surface so primer sticks better. Clean the shower with muriatic acid to further prepare the surface. Rinse the shower, and allow it to dry. Apply the primer using a paint sprayer, and allow it to dry for 24 hours. Finally, spray the shower with two coats of paint.