Grout first, because grout is the sponge in the system: cement grout is porous mortar sitting flush with traffic and splash. Unsealed, it absorbs cooking grease, soap scum, mop water, and spills — which is why grout lines grey and blotch while the tiles around them wipe clean — and in wet rooms it hosts the mould that makes white lines black. A penetrating grout sealer lines those pores: spills sit up for the cloth instead of soaking in, mopping actually cleans, and mould loses its damp anchorage. Minutes of sealing per room, years of easier cleaning.

Tiles — only the porous ones: glazed ceramic and porcelain are effectively sealed by their fired surface — sealer on the glaze adds nothing and can sit as a smeary film (the exception: some polished porcelains take a pre-grouting micro-seal so grout haze releases). Natural stone tiles — travertine, limestone, marble, slate, unpolished granite — genuinely need impregnating sealers, as do terracotta, encaustic/cement tiles (which are essentially decorative grout), and quarry tiles in some settings. The absorbency test settles any tile in a minute: a water drop that darkens the surface marks a candidate for sealing; one that beads indefinitely marks a surface that only needs its grout done.

The room-level summary: seal every cement grout line; seal the tiles too only when the tile itself is porous.

Three product families cover grout's life stages:

1. Penetrating (impregnating) grout sealers — the standard treatment for sound, clean grout: invisible, breathable, no film to wear or peel, water- and oil-repellency per formulation (fluoropolymer versions resist grease best — the kitchen choice). Applied at grouting time (after cure) and renewed periodically. This is what 'sealing the grout' normally means.
2. Membrane/film grout coatings — surface films promising higher stain blocking: legitimate in dry decorative settings, but films on floor grout wear at every traffic line and can peel in wet rooms; the family to choose sparingly and knowingly.
3. Grout colourants/colour sealers — pigmented sealing paints applied line by line: the renovation tier. They recolour blotchy or hopeless grout (any colour, including back to white), seal as they colour, and rescue floors whose grout no cleaner will redeem — at the cost of careful application labour and a maintained-film lifecycle. The honest alternative to raking out and regrouting.

Decision path: new or well-cleaned sound grout → penetrating sealer (fluoropolymer in kitchens/entries); stained-beyond-cleaning but structurally sound grout → colour sealer; cracked, crumbling, or hollow grout → no sealer fixes structure — regrout, cure, then seal the new lines. Epoxy grout, meanwhile, opts out of the whole question: it is non-porous from the bucket and never needs sealing — worth knowing when specifying a new wet room.

A small-brush job that rewards method:

1. Timing on new work: let fresh cement grout cure 48–72 hours minimum (longer per manufacturer) — sealing uncured grout traps water and curbs the cure. On existing floors: deep-clean first with an alkaline grout cleaner, rinse residues fully, kill any mould with the appropriate treatment, and let lines dry 24 h — sealer locks in whatever state it finds.
2. Apply along the lines with the right applicator: brush-tipped or roller-ball sealer bottles trace lines fastest; small brushes and sponge pads work; on fully-porous surfaces (stone + grout), sealing the whole field with an applicator pad is quicker than tracing.
3. The critical habit — wipe the tile: on glazed tile, sealer is for the lines only; haze left on glaze dries to smears. Work a metre or two, then wipe adjacent tile faces with a clean cloth before the sealer flashes.
4. Second coat wet-on-wet or within the stated window on absorbent grout — first coats mostly disappear into thirsty lines.
5. Cure before service: light foot traffic per label (often a few hours), water exposure typically 24–72 h — the shower sealed Saturday morning showers again Sunday night, not Saturday night.
6. Verify: water droplets on sealed lines should sit up or darken only slowly; thirsty spots get another pass.

Whole-bathroom reality check: the labour is the cleaning and drying — the sealing itself is under an hour, which is why doing it at grouting time (clean by definition) is the professional habit.

Working baselines: kitchen and entry floor grout every 1–2 years; shower and wet-room grout yearly; low-traffic floors and wall grout 2–5 years; porous stone tiles (travertine, limestone, marble) 1–3 years in wet or trafficked settings, longer in display ones. Fluoropolymer premium sealers stretch each band; economy sealers compress them.

But test, don't schedule blindly — the water-drop check is definitive: scatter drops along the grout (and on stone): where water sits proud or darkens only sluggishly, protection lives; where it soaks straight in and darkens the line, that zone is due. Test the hardest-working spots — in front of the cooker, the shower's splash wall, the doormat line — because wear is local and resealing can be too: high-traffic lanes annually, the rest of the room on the slow cycle, is a perfectly professional regime.

What shortens the real interval: steam mops and aggressive alkaline/acid cleaners (both strip sealers — pH-neutral routine cleaning is a sealer-longevity measure as much as a stone-safety one), abrasive scrubbing, and outdoor exposure.

The renewal itself is the easy loop: penetrating sealers re-apply over their predecessors after a deep clean — no stripping ritual — which is precisely why little-and-often wins: a fifteen-minute annual top-up in the shower keeps the lines permanently defended, while the skipped-three-years bathroom is back to stain-scrubbing before it can reseal. Sealed-on-schedule grout is the difference between cleaning tiled floors and fighting them.

It changes the fight decisively — but the fight is really about water and air:

What sealing does: mould anchors in damp, porous, nutrient-catching surfaces — unsealed grout is precisely that. Sealed grout starves it: water films drain rather than soak, soap nutrients wipe away, spores find no purchase depth. Showers with sealed grout stay clean-lined for years on ordinary cleaning where unsealed ones blacken in months. Seal only clean, mould-killed, dry grout — treat existing growth with a dedicated mould remover (its roots outlive surface bleaching; the residual-action products beat plain bleach), rinse, dry, then seal, or you laminate the problem in.

What sealing cannot beat alone: a bathroom whose humidity never leaves. Mould's real diet is condensation dwell-time, so ventilation carries half the outcome (extractor run-on after showers, door ajar, squeegee habit on the worst walls — the ninety-second squeegee being the single most effective anti-mould act in any shower).

And the part no grout product touches: silicone joints. The black in corner and tray seals lives behind and inside the silicone where cleaners can't follow — grout sealer doesn't apply, and blackened silicone is a replacement job: strip, clean, dry, re-run with a quality anti-fungal sanitary silicone, which arrives with its own biocide loaded.

The winning regime, in order: mould-treat, seal the grout, replace tired silicone with sanitary grade, ventilate, squeegee. Each step multiplies the others — and the bathroom that does all five simply exits the mould economy.

Same principle — penetrating impregnators — with stone-specific stakes that make product choice stricter:

Why stone raises the bar: travertine, limestone, and marble are acid-sensitive calcium carbonates with visible polished or honed faces; slate and granite vary in porosity; all cost enough that a sealing mistake shows expensively. Stone wants impregnators explicitly rated for natural stone — breathable, non-filming, and (critically on polished faces) non-alteration formulations, unless a colour-enhancing impregnator is deliberately chosen to deepen slate and honed stone tones (test-tile first: enhancement is a commitment). Cheap multi-surface sealers that leave any film create differential sheen on stone faces that only grinding removes.

Practical stone-and-grout workflow, which conveniently converges: on a stone-tiled floor the professional route seals the whole field — stone and grout lines together — with the stone-rated impregnator (often a pre-grouting seal coat on the stone first, so grout haze releases cleanly, then a full seal after grouting). One product, one applicator pad, both porous materials protected; the fluoropolymer stone impregnators serve kitchen grease zones best, exactly as with grout-only sealing.

Stone's companion rules keep the sealer effective: pH-neutral stone cleaners only (acids etch calcium stones through any sealer — the sealer resists stains, not chemistry), prompt wipe-ups of wine/citrus on marble regardless, and reseal on the water-test cadence covered above.

The one-line answer: grout sealer and stone sealer are cousins in the same impregnator family — buy the stone-rated one when stone is present, and it happily does both jobs to the stricter standard.

Glazed ceramic and standard porcelain: no — the fired glaze/vitrified body is tighter than any sealer, and product applied to it just sits as a maintenance-attracting film. The money and effort belong in their grout lines instead. This covers the overwhelming majority of wall and floor tile in modern homes.

The genuine exceptions, worth knowing precisely:

1. Polished (and some lappato) porcelain: polishing can open micro-pores in the vitrified surface; many manufacturers recommend (or factory-apply) a micro-sealer, mainly so grout pigment and metal marks don't lodge during installation — a pre-grouting wipe of porcelain-rated sealer is the classic tiler's step. Check the tile's datasheet; factory-treated ranges need nothing further.
2. Unglazed quarry tiles, terracotta, and encaustic/cement tiles: genuinely porous bodies with real sealing regimes — terracotta traditionally builds its patina under boiled oil/wax or modern matt impregnator systems; encaustic tiles behave like fine decorative concrete and stain readily unsealed. These are their own maintenance cultures, and their suppliers' systems are the safe path.
3. Crackle-glaze ceramics: the decorative crazing network absorbs — sealing before grouting and periodically after is standard, especially in splash zones.
4. Anti-slip structured porcelain: never film-seal it — coatings fill the engineered texture and delete the slip rating (the recurring rule from the anti-slip categories).

Diagnostic once more, because it answers every tile in the shop: water drop on the face — darkens = porous family, sealing regime applies; beads indefinitely = fired-sealed, spend on the grout. The tile aisle's marketing rarely says it this plainly; the water does.