Mostly commonly an epoxy resin filler is applied to the area to fill the damage. Then sand the repaired area smooth. In the case of a large crack in fiberglass, it might require to inject a foam resin beneath the damaged area for support to the repair. Also in such cases, fiberglass mesh along with the epoxy resin filler may be used for further support to the damaged area. The final step then is to match the color of the fixture, and spray a tinted epoxy coating to the damaged area.
Ref: www.permaceramwestchester.com/services/porcelain_fiberglass_
