Zud is the one and only product I’ve ever been happy with for removing rust.
This product claims to remove rust and stains, and to "work where ordinary cleansers fail." I’m here to say that it really does just that.
I used to live in central Indiana where the water coming out of the tap was actually orange. Zud was just a part of everyone’s life! Now the rust in my water is much less of a problem, but it can build up. So out comes the Zud.
This is a powdered abrasive cleanser. If you use it on countertops or plastic, scrub as gently as possible. Do not use on painted or wooden surfaces.
You can see a bathroom sink, somewhat stained in the picture. I wet the surface, sprinkled on the Zud, and waited about two minutes. With light scrubbing with a terry cloth rag, the rust came right off.
It works on fiberglass, but you will have to work a little harder. That said, it works better than anything else on fiberglass. The shower stall is moderately stained, and the picture shows the results after one application (according to each product’s directions) as noted. Each of the areas would need a lot more work, but the Zud is the best.
The hard test is the fiberglass utility sink with real buildup. Again, you can see the results after one application of each product. I know from experience that if you are willing to work at it, you can get almost all the rust out of a utility sink.
Got rust? Get Zud! You may have trouble finding this product, but search it out, or even order on line.