Black stain in bathtub

We have a black stain on the bottom of a procelain bathtub. We thought that this stain was mildew but our old tried and true solution to mildew in the bathroom (a solution of 20% liquid laundry bleach that will eliminate mildew stains from tile grout, tub caulk, etc.) didn't even faze it. Scrubbing doesn't affect it. The stain doesn't cover the entire bottom of the tub evenly, rather it's somewhat mottled in appearance. It's a little rough to the touch. We live in Springfield, MO and have seen a tub in another home with this same type of stain and those folks couldn't get it off either.
Does anyone have any idea how it can be removed.
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We have a black stain on the bottom of a procelain bathtub. We thought that this stain was mildew but our old tried and true solution to mildew in the bathroom (a solution of 20% liquid laundry bleach that will eliminate mildew stains from tile grout, tub caulk, etc.) didn't even faze it. Scrubbing doesn't affect it. The stain doesn't cover the entire bottom of the tub evenly, rather it's somewhat mottled in appearance. It's a little rough to the touch. We live in Springfield, MO and have seen a tub in another home with this same type of stain and those folks couldn't get it off either.
Does anyone have any idea how it can be removed.
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HarryS wrote:

How old is this bathtub? Did the stain appear gradually? Doesn't seem likely but could it be that the porcelain has gone and you are seeing the stained iron? If so, a mild acid such as those used in prepping steel for painting should show you...
Tried a scouring cleanser? Steel wool?
-- dadiOH ____________________________
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HarryS wrote:

I have something similar in shower stall - right where we normally stand. I thought it was likely soil/oil/crud from dirty feet, but has been really difficult to scrub off. Some comes off with 3m and detergent, but it is tough. I've used a lot of CLR, but didn't concentrate on the blob when I used it. Wonder if the glaze can wear a bit, allowing dirt to stick harder?
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wrote:

Toothpaste. With whitener. If that works on a small patch then comet or borax or any other abrasive cleaner.

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HarryS wrote:

Unless the finish is worn (usually very old fixtures) it is likely the result of the local water and maybe the water softener. Sulfur as I recall reacts with several things to cause a black stain.
I don't know how to clean it, but IDing it is a good first step to finding out how and how to prevent it in the future.
--
Joseph Meehan

Dia duit
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