Peroxide + baking soda grout paste
Rex says
Rex Sterling, Ph.D. · Head of R&D, Clean Report · Meet Rex
"This one earns its spot. Oxygen does the work, and the soda keeps it where you need it."
The science
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Oxidation lifts the stain.
3% hydrogen peroxide breaks down into water and oxygen, and as it does it oxidizes the organic stains that darken grout: mildew, soap scum and body oils.
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The paste is the real trick.
Baking soda thickens the peroxide so it clings to vertical grout lines instead of running off. Its mild grit is softer than glazed tile, so it scrubs without scratching.
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A pairing that's actually safe.
Unlike bleach mixes, this pair gives off oxygen, not toxic gas. Mix it fresh, use it right away, and never store it in a sealed container.
Do it right
Spot-test colored grout first. Spread the paste, leave it about 10 minutes, scrub with a soft brush, then rinse thoroughly. Keep it off natural stone.
Where this came from
From the Clean Report hacks libraryJill Nystul · One Good Thing by Jillee
"About 3 parts baking soda to 1 part 3% hydrogen peroxide. Dwell 5–10 minutes, up to 1 hour. Spot-test colored grout."
onegoodthingbyjillee.com/homemade-cleaning-products-recipes/