Should I seal grout, and what with?

Which grouts benefit from a sealer, which do not need one, and the difference between a penetrating sealer and a surface coating.

Difficulty: beginner
Time: 45 min
Read: 3 min

Short answer

Cement grout is porous and benefits from a penetrating sealer in showers and on floors. Epoxy grout does not need sealing at all. Use an impregnating sealer that soaks in rather than a surface coating, apply it to fully cured clean grout, and expect to redo it every year or two in a shower.

Sealing grout does not make it waterproof. It reduces how readily it absorbs water and staining, which slows down discolouration and makes cleaning easier. That is a worthwhile but modest benefit, and it is oversold.

What you'll need

Impregnating grout sealer
A small applicator brush or bottle with a wheel
Clean cloths

Step by step

Find out what grout you have.Standard cement grout is porous and stains. Epoxy grout is dense, non-porous and needs no sealer. If the grout was a two-part mix, it is epoxy.
Decide whether it is worth it here.Shower walls and floors, yes. A splashback behind a kitchen tap, worthwhile. A dry hallway wall, not really.
Wait for the grout to cure fully.New cement grout needs the manufacturer's stated cure, typically one to three weeks, before sealing. Sealing early traps moisture in it.
Clean the grout thoroughly first.A sealer locks in whatever is there. Clean, treat any mould, rinse well and let it dry completely — at least 24 hours.
Choose a penetrating sealer, not a coating.Impregnating sealers soak in and leave no film. Surface coatings sit on top, look shiny, and peel in a wet area, taking a lot of scrubbing to remove.
Apply along the joints, not across the tiles.An applicator bottle with a wheel, or a small artist's brush, keeps it in the joints. On unglazed or natural stone tiles you may want to seal the whole surface instead.
Wipe the excess off the tiles quickly.Sealer left to dry on a glazed tile leaves a hazy film that is unpleasant to remove. Wipe as you go, within the time stated on the bottle.
Apply a second coat where the bottle says to.Most need two coats with a stated interval. Floor grout usually justifies it more than wall grout.
Test it afterwards.Drop water on a joint. It should bead or sit rather than darken the grout immediately. When it stops doing that, it is time to reseal.

Tips

The water-drop test is the only honest way to know when to reseal — annually is a guess, the test is a measurement.
Seal the floor grout in a bathroom even if you skip the walls. It takes far more traffic and staining.
Never seal grout you have not cleaned. You are fixing the current appearance in place.

Common mistakes

Using a surface coating sealer in a shower — The film cannot breathe, water gets under it, and it clouds and peels. Removing it later is a much bigger job than applying it was.
Sealing grout that is still damp or newly laid — Cement grout releases moisture for weeks. Sealing it early traps that moisture, and the grout can stay weak or blotchy permanently.

If it doesn't work

Hazy film on the tiles after sealing

Cause: Excess sealer left to dry on the glaze — Fix: Reapply a little sealer to soften it and wipe off immediately; otherwise a specialist remover.

Grout still stains after sealing

Cause: Sealer worn off, or it was never absorbed — Fix: Do the water-drop test; clean and reapply, allowing full drying first.

Grout looks patchy after sealing

Cause: Uneven absorption over grout of varying density — Fix: A second even coat usually equalises it; it is also a sign the grout was not fully cured.

Questions people ask

Does sealing grout stop mould?

It slows it, because mould feeds on what the porous grout absorbs. It does not stop it — ventilation and drying the surfaces do far more.

How often should I reseal?

In a shower, roughly annually; on a bathroom floor, every year or two; elsewhere, much less. Use the water-drop test rather than the calendar.