This has been a fairly frequent question here in HT. Fiberglass tubs and shower surrounds tend to scratch easy with abrasive cleaners. The result is the gel coating which provides the shiny and surface and makes it easy to clean gets damaged from the abrasives. When the gel coating fails it allows the soils and soap scum to get into the more porous surfaces that the gel coat is supposed to protect. Over time these micro scratches catch dirt, and water hardness resulting in a dingy color
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on the surface. The issue is the only way to remove this is to re-surface the tub/shower area. Using abrasive cleaners will do little to clean and will only make the issue worse. Chemicals such as bleach may lighten up the stains, but will not remove them completely.
Some people have had luck using CLR water hardness cleaner, still others used Fume Free oven cleaner. The biggest thing is you need to let the chemicals sit for an hour or two to let hem do their job.
I just tried this the other day and it works great! Put 1 c. of white vinegar in a bowl and heat it for 2 mins. in the microwave. Carefully pour into a spray bottle. Then add 1 c. Dawn dish soap. Spray on to area to be cleaned and let it sit for 2 hours. Rub with a cloth or gentle sponge and they'll look brand new. I have tried many things over the past 7 years to remove the stains and my shower was pretty badly stained by hard water, so it took a couple of times to clean it, but it looks brand new now!
Since this question is a couple months old, you may have already solved the problem but in the event you haven't, I would like to add that I found Mr. Clean Magic Sponges did a remarkable job of removing those brownish well water stains from a fiberglass tub. It does take some elbow grease (and maybe a couple sponges), but I eventually got the stains off. Also, Walmart has their own brand of the magic sponges and I think they work just as well as the more expensive Mr. Clean brand.
Shari is right. And so is Woolridge. They are iron stains from the well water. I like the idea also of heating the vinegar and some dawn dish soap and use a magic easer to scrub it.
These are a little old, but I have tried all these, none take the stains completely off mine. I have city water, but my mother had well water that had iron in it and she used snow bowl when it would get too bad, took every bit of the stains off. I tried that on mine, but didn't work. My stains are like from the outside in.The tub feels smooth, cleans good with the vinegar and dawn mixture but still has the stains in places.
Some people have had luck using CLR water hardness cleaner, still others used Fume Free oven cleaner. The biggest thing is you need to let the chemicals sit for an hour or two to let hem do their job.