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How do I permanently get rid of this discolouration in my bathtub? (I have hard water)
(sh.itjust.works)
I've tried scrubbing it several times with bar keepers friend soft cleanser and although it improves, it never goes away completely and it always comes back.
Citric acid is commonly used in the canning process. So look in the canning section of the grocery store. The last time I found it was at an ACE hardware that had a very good canning section.
Oxalic acid is an option for rust removal, but it is less common. Basically you need an acid that is safe for fiberglass tubs but works on rust. I have also used stuff like CLR or Lime-Away. Just double check that they are safe for the tub.
Just remember that even weak acids can be dangerous to work with. Wear gloves, eye protection and make sure the room is well vented.
barkeepers friend, which OP already tried, is oxalic acid
Interesting never used the stuff. I wonder what the concentration is?