A few ways
I found this. Hope this helps! https://www.pinterest.com/pin/35606653287888799/
Mix dawn dish soap and vinegar 50/50. Spray on and let it sit..it works miracles
If it is really filthy I use Easy Off Oven Cleaner in the blue can. If the crud isn't only sort of crudy I use a mix of vinegar and Dawn dish soap. Mix them together in a spray bottle. Either way you choose you would best off closing the door after you apply and come back in about 30 minutes.
I always use Washing Up Liquid and a sponge to clean mine. If you have inherited a Scum line, then use a non scratch cleaner Maybe Grunge cleaner. Bet Of luck!
soft scrub works great! It won't scratch like some of the other powder cleaners may.
Here's a post on how I do it - https://chascrazycreations.com/clean-bathroom-natural-products/
Here's what I use:
Fill a spray bottle with a 50/50 solution of Dawn dish soap and vinegar. Spray it on the tub and let it work. If they are tough stains, you may have to scrub with a plastic scrubber!
My favorite thing is "wet & forget shower". It is a solution you spray once a week and it will clean and keep your tub, surround, even glass doors perfectly clean. It isn't a DIY but it's well worth it to me.
If they are really dirty or stained, just scrub with Comet cleanser. Wet the floor and let the cleanser sit for a while to bleach any old stains. Use a brush or Scrub Daddy sponge to rub then rinse.
This should do the trick: https://www.youtube.com/watch?v=_8qeGsaAvi8&t=8s
I use a 32 oz empty water bottle, add 5-6 oz blue dawn dish soap, fill with vinegar. Give it a mix. cut small slits on the top or poke holes, squirt onto the fiberglass tub/shower and let it set for 15 or so min. Take a new soft bristle broom from Dollar Tree (blue bristles) and scrub the tub, rinse with clean water. Repeat as needed.
This information may help you:
Fiberglass is a commonly used material for bathtubs. While it is easy to clean, it can be scratched using the wrong tools and supplies. Combine 2 parts vinegar and 1 part water in a spray bottle and spray the entire bathtub. Allow this soaking solution to sit in the tub for 15-20 minutes.
I have one too and nothing works better than a Mr. Clean Magic Eraser!
Gel Gloss is the best.... https://www.walmart.com/search/?query=gel%20gloss%20cleaner%20and%20polish&typeahead=Gel%20Gloss
Love this stuff https://amzn.to/2IkSPCR