Your porcelain tub is clean. You scrub it every day. But there is still a dull, greyish pattern across the bottom that will not come off — it looks like soap scum, but no amount of scrubbing shifts it. Before you reach for a stronger cleaner, it helps to know what you are actually looking at, because a stain and a damaged surface need opposite treatments.
Most of the time the pattern is soap scum: a film of soap, body oils, and minerals from hard water that builds up in a thin layer and turns dull and grey. It feels slightly waxy and comes off with the right method. But if the surface feels perfectly smooth and the colour is simply there, the glaze itself may be worn or stained underneath the surface — and no cleaner will fix that. Here is how to tell the difference and what to try, in order.
Quick overview
- What it is: A step-by-step routine for removing stubborn discoloration and soap-scum patterns from a porcelain tub.
- Time needed: 20 minutes of active work, plus up to 30 minutes while pastes sit on the stain.
- What you need: Baking soda, mild dish soap, hydrogen peroxide, a soft sponge or non-scratch pad, rubber gloves, and a microfibre cloth.
- The rule: Start with the gentlest method and work up. Porcelain glaze is tough but it can be dulled by years of harsh scrubbing, and a dull glaze holds dirt faster.
First: the fingernail test
Run a fingernail lightly across the pattern. If you can feel a slightly raised or waxy film, it is sitting on the surface and you can probably remove it. If the surface feels perfectly smooth and the colour is flat and even, the discoloration is in the glaze or underneath it, and cleaning will lighten it at best.
Also look at the shape. A pattern that follows where water sits, or where a bath mat used to be, points to soap scum and mineral deposits. A patch that is rough to the touch, or where the shine is gone, points to wear in the glaze itself.
Step 1: Baking soda and dish soap paste
Mix baking soda with a few drops of mild dish soap and a little water until it forms a thick paste. Spread it over the pattern and leave it for 15 minutes. The baking soda is a mild abrasive, gentle enough for porcelain, and the dish soap cuts the greasy part of soap scum.
Then scrub gently in circles with a soft sponge or a non-scratch pad. Rinse with warm water and dry the area with a microfibre cloth — drying matters, because water left on the surface leaves mineral spots that look like a new stain. For a normal soap-scum pattern, this step alone often does the job.
Step 2: Baking soda and hydrogen peroxide
If the pattern is still there, make a second paste: baking soda with hydrogen peroxide (the standard 3% kind from the pharmacy) instead of water. Spread it over the stain, leave it for 20 to 30 minutes, then scrub gently and rinse well. The peroxide helps lift mineral and dye stains that soap alone cannot shift.
Do not mix hydrogen peroxide with vinegar or bleach. Use it on its own, with gloves, and rinse the tub fully before anyone uses it.
Step 3: Bar Keepers Friend, used sparingly
If the first two steps did not work, Bar Keepers Friend is the next step up. Its active ingredient is oxalic acid, which dissolves mineral deposits and rust, and it is generally safe on porcelain when used correctly. But it is an abrasive cleaner, so the technique matters more than the product.
Wet the surface, sprinkle on a small amount, and rub gently with a soft sponge. Rinse it off after one minute — do not leave it sitting on the surface, because leaving it on too long can cause discoloration. Test it on a hidden spot first, and use it sparingly: repeated heavy scrubbing with any abrasive will dull the glaze over time, and a dulled glaze stains faster than a shiny one.
Do not use Bar Keepers Friend on marble, gold-plated fixtures, or painted surfaces. And never use scouring powder, steel wool, or a green scouring pad on porcelain — they will scratch the glaze, and the scratches will fill with dirt within weeks.
When it is damage, not dirt
If you have worked through all three steps and the pattern looks exactly the same, it is probably not dirt. Enamel can wear thin after years of use, and once the glaze is worn the colour underneath shows through. A bath mat left in place for years can also leave a permanent shadow where the covered area aged differently from the rest of the tub.
Neither of those can be cleaned away. The real fix is refinishing the tub, which is a professional job. A short-term cover is a non-slip bath mat, which at least hides the pattern. Do not be tempted to keep scrubbing harder — you will only wear the glaze down further.
Keeping it from coming back
Soap scum forms when soap mixes with minerals in hard water, so the cheapest prevention is to rinse the tub after every bath and dry it with a quick wipe. A squeegee or a microfibre cloth takes 30 seconds. Use liquid soap rather than bar soap if you can — bar soap leaves more scum. And give the tub a proper clean once a week, before the film has a chance to harden into a pattern again.