Should I seal a patio or block paving?

What a sealer does and does not do, which paving benefits, and the preparation and timing that decide whether it works.

Difficulty: beginner
Time: 3 hr
Read: 4 min
Safety: caution

Short answer

Sealing is worth it on porous sandstone and on block paving that keeps losing joint sand. It is not worth it on dense stone or porcelain. Whatever you use, the paving must be scrupulously clean and completely dry, and the first year's efflorescence must have finished.

A sealer is not a protective skin. Most are impregnating treatments that reduce how much water and dirt the stone absorbs, which makes stains easier to remove and slows algae. They do not stop weeds, they do not stop the paving getting dirty, and applied to damp or dirty paving they lock the problem in permanently.

Safety

Many paving sealers are solvent based, flammable and give off strong vapour — do not use them in an enclosed area, keep them away from ignition sources and follow the ventilation guidance. Sealed paving can be significantly more slippery when wet. Keep sealer out of drains and off planting.

Step by step

Work out whether the paving actually needs it.Splash water on a dry slab. If it soaks in and darkens within a minute, the stone is porous and a sealer will help. If it beads or sits, sealing achieves very little.
Wait for new paving to finish efflorescing.New paving pushes salts to the surface for the first year or so. Sealing over that traps them under the sealer where they stay. Wait a full year on new work.
Clean thoroughly and let it dry completely.Any dirt, algae or stain under the sealer is there permanently. After cleaning, allow several dry days — most sealers specify the paving must be dry to depth, not just on the surface.
Check the joints are sound first.Repoint or rejoint before sealing. Sealer over a failing joint does not hold it together and makes later repointing harder.
Test on a spare slab or a hidden corner.Sealers change the colour of stone, sometimes dramatically. A wet-look sealer on a pale sandstone is a very different patio. Test and wait for it to cure before committing.
Apply thinly and evenly.Two thin coats rather than one heavy one, with a low-pressure sprayer or a lambswool applicator. Pooled sealer dries to a shiny patch that has to be stripped.
Watch the weather window.Most need several dry hours to cure and a minimum temperature, often 10C. Rain during curing ruins the finish. Spring and early autumn are usually the best windows in the UK.
Plan the reapplication.Impregnating sealers last three to five years; film formers less in traffic areas. Diary it, because a sealer that has partially worn away looks worse than none.

Tips

Never seal wet paving, and remember that stone can be dry on top and damp 5mm down after rain a day earlier.
Sealed paving is more slippery in the wet. If the patio is on a route people use in all weathers, choose a matt impregnating sealer rather than a wet-look one.
A joint stabiliser on block paving solves the sand-washout problem without changing the appearance much, and is a smaller commitment than a full sealer.

Common mistakes

Sealing paving that has not been properly cleaned — Every mark, algae stain and cement smear is fixed under the sealer and can only be removed by stripping the sealer off, which is a much bigger job than cleaning was.
Sealing new paving in its first year — Salts still working their way out of the material get trapped under the sealer and cause a permanent white bloom beneath the surface.

If it doesn't work

White cloudy patches under the sealer

Cause: Sealed over damp paving or trapped efflorescence — Fix: The sealer has to be stripped with the manufacturer's remover, the paving dried thoroughly, and the job redone.

Sealer wearing off in traffic lanes

Cause: Film-forming sealer on a busy route — Fix: Strip and switch to an impregnating sealer, which wears far less visibly.

Patio now dangerously slippery

Cause: Wet-look or film sealer — Fix: Some sealers accept an anti-slip additive on reapplication. Otherwise strip and use a matt impregnating product.

Sealer dried patchy and glossy in places

Cause: Applied too thickly and pooled — Fix: Two thin coats is the method. Pooled areas need stripping and redoing.

Alternatives

Impregnating sealer: Soaks in and leaves no film. Reduces absorbency, keeps the natural look, and is the safest general choice for natural stone. Reapply every three to five years.
Film-forming sealer: Leaves a surface layer, often with a wet look or a sheen. Enhances colour and binds joint sand, but it wears in traffic lanes, can go cloudy, and is difficult to remove once it starts to fail.
Joint stabiliser for block paving: Binds the kiln-dried sand in the joints so it stops washing out. Genuinely useful on a drive that loses sand every winter.
No sealer: Clean annually and accept some weathering. Perfectly reasonable, especially on dense granite, porcelain and concrete flags where absorption is low anyway.

Questions people ask

Does sealing a patio stop weeds?

No. Weeds grow in the joints, in debris that settles there. A sealer on the slab faces makes no difference — full-depth jointing does.

How long does patio sealer last?

Three to five years for an impregnating sealer, often less for a film-forming one in a traffic area. Reapplication needs the same cleaning and drying preparation as the first coat.