A hydrogen chloride base product will prob take it off, but it's going to take the paint with it.
A janitorial supply company in your area probably has an industrial product available, maybe with a name like Lime Remover.
A Phosphoric Acid based product might work as well, although it will not be as aggressive, and will probably take the paint off as well. Common phosphoric acid based products are Lime Away, available at any store.
More concentrated products would be available at a Janitorial Supply company in your area.
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