Oxalic acid is a key ingredient in several commercial cleaners that advertise their suitability for cleaning grout. Care needs to be taken when using these products because oxalic acid can actually damage granite and marble, rust metal fixtures and eat away grout if used too frequently and not properly rinsed. As a last resort, however, and with proper precautions, oxalic acid can be particularly helpful in restoring grout stained by lime and rust.
Make a paste the consistency of peanut butter with an oxalic acid cleaner, like Bar Keeper's Friend, and water. With thinner cleaners, you'll have to gently dab the full-strength solution onto the grout with a rag. Though oxalic acid is less corrosive than bleach, you should still wear gloves while working with it.
Wet the grout and surrounding tiles before applying the oxalic acid cleaning compound.
Rub the cleaner paste into the grout between the tiles. Allow it to sit for no more than a minute or two, especially if you are working with marble or granite. Oxalic acid creates an acid slurry on the surface of grout or some stone tiles. This dissolves surface stains that have bonded with the grout.
Wipe the areas around the grout to avoid damage to the tiles, especially stone tiles. Wipe with large circular strokes, especially with stone tiles as the oxalic acid will have a polishing effect on surrounding tiles. Large strokes even the effect and help prevent blotching of the grout. Rinse several times thoroughly, drying the surface with towels to remove all of the cleaning product.
Seal the grout with grout sealant according to manufacturer's instructions. The sealant will help preserve the restored colour of the grout and help prevent having to use this cleaning strategy too frequently. Too frequent use of oxalic cleaners can result in blotching of grout as the acid eats away the surface of the grout over time. Penetrating sealer works better than topical sealers and needs to be reapplied annually to maintain protection.
Oxalic acid cleaners can also remove rust from metal fixtures as well, using the same cleaning precautions. Oxalic acid is found in foods like spinach and is a more natural cleaning agent than many other chemicals on the market. But due to its concentration, oxalic acid should be treated with the same care and respect with which you would treat bleach or muriatic acid.
Oxalic acid eats away limestone and similar minerals. That's one reason it works so well and another reason you should only use it as a last resort. Always wear eye and hand protection when working with chemical cleaners. Test the tiles first with a mild acid in an inconspicuous area to make sure the cleaner won't damage the tiles. Do not mix oxalic acid products with other cleaning products to avoid the release of toxic fumes. Don't use oxalic acid cleaners on gold, silver, pewter, marble, lacquered metals, or anodised aluminium.