How to clean an old fiberglass tub/shower?

The first thing I would try would be to make a paste with water and baking soda. Make it so you can brush it on or rub it on with you hand. Leave that until it starts to dry a bit. Then I'd spray it with a mixture of 50% water & 50% vinegar. Let it fizz and bubble. When it stops, wipe down with a sponge, then if you have a hand-held shower use that as a final rinse. If you don't have a hand-held shower, get a bucket you can handle, put water in it and rinse it that way.

Good luck.

I had the same problem. CLR makes a toilet cleaner, its very thick so it will stay on the surface. Stinks and bubbles so open windows and wear gloves!

Mix one part vinegar with one part dawn dishwashing liquid. Spray this mixture on the entire surface, and let soak...then scrub and rinse. You will be amazed how much scum and grime comes off. After scrubbing and rinsing, you may want to spray with a mist of bleach water to help sanitize everything as well

Wet dryer sheets with vinegar, apply to service like your wallpapering, and let sit a couple of hours or overnight. Use dryer sheets to them scrub the walls clean and then rinse.

Try Easy Off No Fume Oven Cleaner in the BLUE can. Spray it on, let it foam and sit until it liquefies. Then rinse away. Works on porcelain and fiberglass.

Good luck!