Fiberglass shower pans have increased in popularity due to their ease of installation, light weight structure and inexpensive sale price. After months of having water and soap sitting in it, the shower pan can have some nasty crud built up on the corners and edges as well as strange colored stains. Replacing the shower pan is a complicated process and unnecessary if you can get it to be fresh and clean.
Identify the type of buildup you have. If it is dark and greasy, it is probably just dirt and greasy film often left over from bar soap. If it is yellow and very hard, you probably have a scale buildup from calcium, magnesium and other minerals left during water evaporation. If it is bluish green or orange you have iron or copper deposits, either from your pipes or from the water itself.
Apply a grease cutting soap such as Fantastik Clean and Shine for soap scum residue. It will cut through the grease, emulsify it and allow you to wipe it away with a lint free microfiber cloth. If it proves to be a little stubborn from weeks of buildup, use a plastic sponge scrubber to scrub it clean. Rinse and wipe dry.
Remove calcium and magnesium buildup by applying an acid-based cleaner to dissolve the minerals. One product made specifically for removing this type of buildup on fiberglass is CLR Bath and Kitchen Cleaner. It is formulated not to etch the fiberglass surface as some stronger acids might.
Dissolve iron and copper stains by applying a bleaching product like Bar Keeper's Friend. It is safe for fiberglass if applied with a plastic sponge as a paste. You might have to leave it on for a few minutes before scrubbing, but it removes most metal deposits.
Freshen the fiberglass shower pan with a dilution of bleach and water sprayed on with a plastic spray bottle. A product made specifically for this application is Clorox Clean-Up Cleaner with Bleach. You spray it on, wait 30 seconds and wipe or rinse the shower pan clean.