Mix baking soda with water to form a thick paste.
Apply the paste to grout lines.
Scrub grout lines with a stiff-bristled brush.
Rinse with clean water and wipe dry.
For tougher stains, spray grout lines with a 50/50 solution of white vinegar and water.
Let sit 5–10 minutes.
Scrub grout lines thoroughly.
Rinse well and wipe dry.
For heavy mildew, apply hydrogen peroxide to grout lines.
Let sit 10–15 minutes.
Scrub and rinse thoroughly.
Use an old toothbrush or grout brush for corners and tight spaces.
Avoid saturating porous grout or surrounding materials.
Rinse and dry completely after cleaning.
After grout is fully dry, apply a grout sealer to help prevent future staining.