How to Remove Mildew from Grout

The Logic Verdict

My Take: Laura only needs to rescue five or six vertical grout lines, so she mixes about a tablespoon of washing soda with just enough bleach to make a stiff paste. She spreads it with a detail brush so it clings instead of running, leaves it alone for roughly 20 minutes, and the mold disappears without scrubbing or fogging the bathroom with bleach overspray.

The Science

Washing soda (sodium carbonate) is far more alkaline (≈pH 11.5) than baking soda (≈pH 8.5), so it breaks down the biofilm and soap scum that shield mildew colonies. Adding a splash of sodium hypochlorite bleach oxidizes the pigment that makes the grout look black. Keeping the mixture as a paste—rather than diluting it into a spray bottle—lets the chemistry sit at full strength on the stain while avoiding bleach mist that can pit stone or choke the room.

Step-by-Step Removal

1. Ventilate and gear up. Pop a window or the bath fan on and pull on gloves; even though she uses only a few milliliters of bleach, Laura still respects the fumes.
2. Mix a thick paste. Measure about 1 tablespoon of washing soda into a small container and drip in bleach until it’s the texture of peanut butter. You only need enough to coat the problem joints.
3. Load the grout lines. Use a grout/detail brush or old toothbrush to butter the paste onto the discolored joints. Aim for a wet layer that clings to the lines without dripping down the tile; scoop the excess back into the cup.
4. Let it dwell for 15–20 minutes. Resist the urge to scrub. The alkaline/bleach combo lifts the mildew while you wait.
5. Rinse and inspect. Rinse the walls with the shower head, then buff the joints with a microfiber cloth. Repeat a light application only if you still see shadows.

Parts & Tools

• Arm & Hammer Super Washing Soda (Amazon) – Provides the high-pH base Laura mixes with bleach.
• OXO Good Grips Deep Clean Brush Set (Amazon) – Narrow, stiff bristles make it easy to spread the paste on just the grout lines.

What NOT To Do

• Don’t pour bleach straight from the jug down the grout lines—the author limits herself to roughly 5 mL to avoid overpowering fumes.
• Don’t thin the solution into a spray bottle; atomized bleach is harder to control and can etch natural stone or metal trim.
• Don’t leave the paste on for hours; 20 minutes is enough to lighten the stain without softening surrounding grout or caulk.

Resources

• Arm & Hammer Super Washing Soda (Amazon)
• OXO Good Grips Deep Clean Brush Set (Amazon)