Title: Use Vinegar instead!
Review: Think about it. If the stain is from acidity in the water taking copper from the pipes, say, and depositing it under the tap (or the like) - vinegar will wash it away in a trice. There may be other great uses for this pumice stone - but I was just scratching my porcelain bathtub unnecessarily. Someone I talked to had read a book on the chemistry of cleaning and gave me the tip. The blue washed away easily.
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