Fiberglass Shower Stain Remover & Revitalizer Kit

The user is struggling with 'hard dark stain' on a fiberglass shower that conventional cleaners and vinegar only partially remove with 'a lot of scrubbing.' This indicates a need for a specialized product or kit designed for fiberglass that can effectively and safely remove hard water stains and dullness, restoring the 'smooth and shiny' finish. The kit could include a powerful, fiberglass-safe cleaning agent, an appropriate applicator/scrubber, and potentially a sealant/polish to prevent future staining.