>>2986094
I usually use a razorblade followed by an acid based cleaner like CLR. I'm sure there are other products that work better. If you have Mable or limestone the hard water cleaners will destroy it. After it's clean, start using a squeegee when you shower and get a softener.

Where's the M U R I A T I C A C I D guy?
Anyway, citric acid.

>>2986094
Vinegar or lemon juice or if you want to be fancy dilute some lactic acid.

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>>2986135
This is not very effective though.