Baking soda paste, ten minutes, gentle scrub. That is the whole secret to white shower grout, and I found it only after scrubbing one line with a toothbrush for twenty minutes and getting nowhere. The black stuff is soap scum and mildew, and plain soap does not touch it.

The paste that actually works

Make a thick paste of baking soda and a little water, spread it over the grout lines, and let it sit for ten minutes. Then scrub gently with an old toothbrush or a grout brush. The paste does the lifting, so you do not have to grind the brush into the tile.

For the darker mildew stains, I follow the paste with a spray of white vinegar and let it fizz. Vinegar cuts the soap scum and kills a lot of the mildew on contact. Just do not use vinegar on natural stone tile, it can etch it.

Where the black stuff hides

Grout is porous. Mildew grows in the pores and stains the whole line, not just the surface. That is why wiping the surface with a sponge does nothing. The cleaner has to sit on the grout long enough to soak in.

My routine now:

1. Apply the baking soda paste at night.
2. Let it sit while I do other things.
3. Scrub lightly and rinse.
4. For stubborn lines, repeat with the vinegar spray.

Keep the bathroom fan running and the window cracked while you work. Grout cleaners are not perfume.

Keeping it clean longer

The real fix is drying the shower. Mildew needs moisture to grow, so I squeegee the walls after every shower and leave the curtain open so air moves through. It takes fifteen seconds and cut my grout scrubbing way down.

Once the grout was clean, I also stopped letting shampoo bottles sit on the floor where water pools behind them. Less standing water, less mildew, less scrubbing.

Two months later the grout is still white. The toothbrush era is over.