In extreme cases you may have to wash with a mild solution of muriatic acid.

Test a mild solution of 3% hydrogen peroxide and water in an inconspicuous area - if safe, apply gingerly to the stain.

A milder solution can be made by mixing a cup of borax with the detergent solution used for washing.

Recently, I attempted to clean a sanded grout joint with a mild solution of muriatic acid and water.

Sulfuric acid is flushed with a mild, soapy solution if the burns are not severe.

Otherwise wash the bricks with a mild solution of muriatic acid.

Clean out the fridge every week or two, using a mild solution of bleach and water.

If that does not work, wash the wall with a mild solution of muriatic acid, which is available at building supply stores and home centers.

Any classical solution is a mild solution.

For polished cotton, another cleaning challenge, he recommended wet cleaning with a mild solution so the shine won't come off.