ChatterBank21 mins ago
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Try a very dilluted mix of warm water with muratic acid, which we use to clean concrete. The acid is available at most DIY stores here in the U.S. I'd mix aboutt 1/4 cup of the acid to about 2 quarts warm water as a starter and even then add just a little to a corner of the stain to see how it will react… brush lightly with an ordinary broom and rinse thoroughly. You might need to make the mix a little stronger but don't let it sit too long...