my fibergalss unit has yellowed in spots and I'd like to know if there's a way to make it white again.
Not sure you can, but try UPVC cleaner it might just help! Best wishes.
My friend used oven cleaner with good results
You can also use Magic erasers. They are slightly abrasive so they actually sand off the yellowed finish. This will take a LOT of elbow grease and erasers, but I have found it to make some difference. I don't think you will ever get it back to like new, though.
Lysol Lime & Rust Remover Toilet Bowl cleaner, comes in a black squirt bottle. Scrub with Scrub Daddy or Mr. Clean Magic Eraser. If stubborn, use a scouring stick to scrub.
Sadly, it could be the finish actually yellowing so be careful not to remove the finish in your cleaning efforts.
I also tried the oven cleanser, Easy Off , recommended from this site. It works! I was going to have to resurface my tub. You need good ventilation and wear a mask.