When I moved into my apartment, I assumed the grout in the bathroom was supposed to be dark gray. It was consistent — dark gray lines between white tiles, evenly colored from wall to floor. I thought it was a design choice.

Then I dropped a wet white washcloth on the floor and when I picked it up, there was a bright white streak in the grout underneath. The grout was white. It was just buried under years of dirt.

I spent a weekend on my hands and knees and brought it back. Here’s what worked.

The baking soda and hydrogen peroxide paste

Mix baking soda with hydrogen peroxide to make a thick paste — about the consistency of toothpaste. Hydrogen peroxide is a mild bleach and disinfectant. Baking soda provides the grit for scrubbing without scratching.

Apply the paste to the grout lines with an old toothbrush. Let it sit for about 10 minutes. Then scrub. The paste will foam slightly as the peroxide reacts. Scrub in small circles along each line.

Wipe away the paste with a damp cloth. The difference is immediate and kind of shocking. What looked like dark grout is actually just grime that regular mopping never touches.

For really stubborn grout

If the paste doesn’t get it all — and it might not if the grout has been neglected for years — try oxygen bleach powder (not chlorine bleach). Mix the powder with warm water according to the package, apply it to the grout lines, and let it sit for 15-20 minutes. Then scrub and rinse.

Oxygen bleach is color-safe and won’t damage most tile. Chlorine bleach can discolor colored grout and etch certain stone tiles, so skip it unless you’re absolutely sure your tile can handle it.

Steam if you have one

A steam cleaner with a narrow nozzle attachment is the nuclear option. The high-pressure steam blasts dirt out of the porous grout without any chemicals at all. If you already own one, use it. If you don’t, start with the paste — steam cleaners are great but the paste costs about fifty cents per application.

Once the grout is clean and dry, apply a grout sealer. This is the step most people skip because they’re exhausted from cleaning, but it’s the one that makes the cleanup last. Sealer prevents dirt and moisture from soaking back into the grout. Apply with a small brush or the applicator bottle it comes with, let it dry, and your grout will stay clean for years instead of months.

My bathroom looked like a different room afterward. I kept going in there just to look at it for about a week.