Alkaline deposits in the water cause those not so pretty spots on your glass doors and shower walls. Apply a car wax to the walls and doors, but not the floor or it gets real slick. Both fiberglass and glass are porous. The car wax seals those pores, which makes cleaning easier and keeps water spots away. Reapply twice yearly.
Squeegee or wipe down your shower after each use to further prevent buildup. If you're tired of fighting the soap scum buildup, switch to liquid soap, natural soap or Dove. It's the talc in most bar soaps that causes the buildup. Changing soap can eliminate the buildup. You still need to clean the shower once a week, but the job is easier.
Clean the showers once a week with an orange citrus based cleaner. Spray on and give it ten minutes to dissolve the dirt. Why do all that scrubbing when your product does it for you?
OK, so the soap scum refuses to budge and friends are coming for a visit. Use your orange based cleaner concentrated. Pour plenty on an old dishrag and wipe on the walls and doors. Use can use boiling hot white vinegar (wear gloves) but you must keep applying it as the white vinegar dries. It takes time to dissolve that buildup.
Patience now becomes a virtue. Wait and wait and wait some more. Go clean the rest of the house. If the cleaner starts dripping down the walls, wipe it back on. Check the walls with a fingernail. If the residue removes easily, round one is about over.
Use a white bristle pad to scrub. They look like fibers and are glued to a sponge. Use the white ones only. The colored scrubbers are coarser and do scratch. Test a spot to make sure it won't scratch the fiberglass. Dampen the pad, keeping it good and wet and gently scrub. This removes the soap buildup and most of the white mineral deposits on the glass doors. Nothing removes the etch marks themselves, but further damage is halted. Reapply the orange citrus cleaner if necessary.
Plastic shower curtains can be machine washed. Remove, spray with your diluted orange based cleaner and wait about 10 minutes. Wash with your rags and they come out quite clean. Air dry. If the curtain is long enough, cut off the seam at the bottom to prevent mildew buildup down there.