How do I get my fiberglass tub white again

Have you tried using oven cleaner on your fiberglass? I used it all the time and it cleans beautifully and effortlessly. Never had to whiten but it might be worth a try.

http://verymom.com/2014/01/26/my-fiberglass-bathtub-is-gleaming-again-heres-how/

I use baking soda and peroxide mixed into a paste on our tub which is acrylic - it feels like a tupperware tumbler

Easy Off No Fume Oven Cleaner in the BLUE can. Works on enamel, porcelain, and fiberglass.

If age has turned the color of your shower stall or tub surround to a uniform insipid yellow, you probably won't be able to restore the color with a cleaner. Instead, purchase a tub and shower refinishing kit from a hardware store or building supply outlet. The kit contains paint specially designed for use on acrylic and fiberglass as well as chemicals to prepare the plastic. Follow the instructions in the container when using one of these products. It's important to wear a respirator and provide ventilation in the space in which you're working, because the chemicals are volatile and noxious.

This is a great example....http://www.domestiphobia.net/2013/02/05/how-to-make-your-nasty-yellow-fiberglass-tub-look-new-for-50/

Maybe some coke a cola in a spray bottle or some white vinegar. Might work!

Put the shine back in your porcelain sinks and bathtubs by giving them a good scrubbing with full-strength white vinegar, followed by a rinse of clean cold water. To remove hard-water stains from your tub, pour in 3 cups white vinegar under running hot tap water.

I've tried EVERYTHING! I have extremely rusty water and found this tip on Pintrest. I used the EZ Off oven cleaner in the BLUE can. I let it set overnight then used my battery operated scrubby with the long wand. I have a fiberglass shower and this worked like a miracle!! Blessings to you and hope you have the same good results. Rust is the toughest stain to remove, but others used it for the dingy look as you have. Please respond to learn how this worked for you!

Oven cleaner does work, but it may discolor your fiberglass.

There is a product we bought called Rog3 and it was the only thing that worked. Ours was blackened around the raised area (slip resistant). It was not yellowed like yours. But it was the only thing that worked. We tried oven cleaner, magic sponge, bleach products. Check out the web site, it is the only place you can get it, www.rog3.com

Several years ago, I managed apartment blds. and we had a problem similar to yours. I tried everything. Tubs needed to be replaced and at that point I had nothing to lose. I bought a spray bottle of Bath and Kitchen, sprayed the tub, let it set for 15-20 min. took a scrub pad and that awful stuff came off but, it was like a blackened hard water stain. I'm not sure about yellowing. Tubs were almond color but they looked brand new.

I sure hope this helps you with the color, but I guarantee you it gets it really clean and won't hurt it. Let me know if it works or not. Good luck.