My bathroom floor has white grout. Whoever chose white grout for a bathroom floor should be forced to clean it themselves for a year. After six months of ignoring it, the grout lines looked like someone had drawn them with a dirty pencil.

I tried a scrub brush. I tried a “grout eraser” from the cleaning aisle. I even tried one of those drill brush attachments that influencers love. All of them required serious elbow grease and the results were uneven. Then a friend who used to clean houses professionally told me the toothbrush trick — and it does not involve scrubbing.

Make a paste, let it sit, let chemistry do the work

Mix baking soda and hydrogen peroxide into a paste. Not a liquid — thick enough to sit on the grout line without running. The consistency of yogurt. Spread it along the grout lines with an old toothbrush. Let it sit for 15 to 20 minutes. This is the part I always used to skip — I’d apply and immediately scrub. The dwell time is where the magic happens.

The hydrogen peroxide breaks down the organic gunk (soap scum, skin cells, yes it’s gross) and the baking soda provides gentle abrasion. After 15 minutes, come back with the toothbrush and work it in a little — not scrubbing like you’re mad at it, just agitating the paste. Then wipe everything away with a damp cloth.

The first time I did this, I actually laughed. The grout went from dark gray back to off-white and I had barely used any muscle.

For the really stubborn dark spots

If some lines are still dark after the baking soda treatment, upgrade to oxygen bleach powder (the kind sold as “OxiClean” or generic equivalent). Mix it with just enough warm water to form a paste. Apply with the toothbrush, let it sit for 30 minutes, then rinse.

Do not use chlorine bleach on colored grout — it will strip the color right out. Oxygen bleach is color-safe. Also: do not mix peroxide and bleach. I shouldn’t have to say this but I will anyway because the internet has some truly dangerous “cleaning hacks.”

Keep it clean once you’ve restored it

After you put in the work, seal the grout. Grout sealer costs about eight bucks and takes fifteen minutes to apply with a little brush bottle. It prevents moisture and dirt from penetrating. Sealed grout stays clean for a year; unsealed grout looks dirty again in a month.

Also: squeegee the shower floor after the last person showers. Water sitting on grout = mildew food. It takes ten seconds and makes a real difference.

• Paste: Baking soda + hydrogen peroxide, yogurt consistency
• Dwell time: 15-20 minutes — this is the most important step
• Stubborn spots: Oxygen bleach powder paste, 30 minutes
• After cleaning: Seal the grout with grout sealer
• Maintenance: Squeegee shower floor after use