Outdoor tile sealing is sold as a blanket service and it should not be. Some outdoor tiles need sealing, some need only the grout sealed, and some need nothing at all — and sealing those is actively counterproductive.
Start with what the tile is
Porcelain. Fired at high temperature to near-zero porosity — typically under 0.5% water absorption. It does not need sealing, cannot absorb a penetrating sealer, and a film-forming sealer on porcelain sits on top, goes cloudy, scratches and eventually peels.
If someone quotes to seal your porcelain tiles, that is the test question. The grout between them is a different matter.
Natural stone — sandstone, limestone, travertine, bluestone. Porous, and sealing is genuinely worthwhile. Breathable impregnators only.
Terracotta and unglazed quarry tile. Very porous, needs sealing, and traditionally with a breathable product.
Glazed ceramic. The glaze is the seal. Grout only.
Concrete and reconstituted tiles. Porous, benefit from sealing.
The slip problem
This is the part that matters most outdoors and gets least attention.
Outdoor tiles are specified with a slip resistance rating, and that rating assumes the surface as supplied. A film-forming or wet-look sealer fills the surface texture and reduces grip, sometimes dramatically.
Around a pool, on steps, or on any sloped area, that is a real hazard and a liability question — slip resistance and who is liable.
Impregnating sealers do not change the texture, which is one more reason they are usually the right answer outdoors.
Impregnating versus topical
Impregnating (penetrating). Soaks in and lines the pores. No change in appearance, no film, nothing to peel, and the tile still breathes. Five to ten years, and reapplication is just clean and recoat.
Fluoropolymer versions resist oil as well as water, which matters near a barbecue or an outdoor kitchen. Silane-siloxane is fine elsewhere.
Topical (film-forming). Sits on the surface, usually acrylic. Enhances colour, adds gloss, reduces slip resistance, traps moisture, and peels when it fails. Outdoors in Sydney UV this is rarely the right choice.
The general argument is in choosing a concrete sealer.
Grout
Often the real reason to seal. Cement grout is porous and sits lower than the tile, so it collects water and dirt and grows mould first.
A penetrating grout sealer is worth doing even where the tile needs nothing — and on a porcelain patio that is the whole job.
Epoxy grout needs no sealing at all.
Why the joints fail before the tile does is covered in outdoor tile and grout.
Before sealing
The tile has to be clean and dry, and on outdoor tile both are harder than they sound.
Clean means growth and staining actually gone, not bleached pale. Sealing over mould locks it under a product that will last years.
Dry means dry through, which after a wash in Sydney humidity is several days, not one. Sealing damp tile traps moisture and the result goes hazy.
And no acid on natural stone during the clean — it etches carbonate stone permanently. See how to clean outdoor tiles.
Testing whether it has gone
Flick water onto the tile and the grout separately. Beading means the sealer is working. Darkening and absorption means it has gone.
Grout usually goes before the tile does, and can be resealed on its own.
When not to seal
Porcelain. Glazed ceramic. Any tile where moisture is coming up from below — sealing the top of a surface with a drainage problem traps the water and causes the tiles to lift.
And any surface where slip resistance is critical and the only product on offer is a film.
For cleaning and sealing as one job, see tile and grout cleaning.
