Hi Mary -
You know, I ran out a couple of months ago and never replaced my laundry rust remover yet - I'm shopping tonight and will have to check. In the meantime, I get a catalog called "The Clean Team" also found at www.thecleanteam.com
They carry a rust remover product with this explanation at http://www.thecleanteam.com/catalog_f.cfm
"Finally there's a better alternative to those other harsh rust removers. Our Rust Remover is a biodegradable and organic formula that overpowers tenacious rust stains without caustic, irritating chemicals or fumes. Use it on countertops, sinks, faucets, window frames ... even carpets, upholstery, and laundry. This easy-to-use, fast-acting formula never ceases to amaze."
(In their Q&A I found this:
Examples of products with oxalic acid are "Rust Remover" (our product), Barkeeper's Friend, and ZUD. Oxalic acid is toxic, so follow the manufacturer's directions respectfully and completely. Don't use chlorine bleach on rust because it sets the stain and actually makes your problem worse.
THEY ALSO HAVE A FIBERGLASS CLEANER:
Fiberglass keeps a death-grip on grime, yet you can't use abrasive cleaners on it or the poor sensitive thing will scratch. Most fiberglass cleaners we've tried (up to and including serious formulas for boats) claim a lot but do little other than work up a sweat and frustrated expectations. Our Fiberglass Cleaner removes scum and soap build-up like a dream. Also great for Corian, all kinds of marble, and outdoor furniture.
I have another product called "Simple Green" you may have heard of - on the label it says it is safe to use on fiberglass.
You asked how the laundry rust remover worked on my shower - it worked beautifully! I did not detect any "erosion" or damage to the surface of the fiberglass - I use it about once a year on the fiberglass shower and marble sink - kind of a "spring cleaning" ritual.