Preparing Block Paving Before Applying Sealant

Why the Prep Decides How the Sealant Performs

A block paving sealant is only ever as good as the surface underneath it. Applied over damp, dirty or shifting paving, even a premium product will lift, cloud or peel within a single winter — and it will take the stain of every oil drip and moss spore with it. Done properly on a dry, sound, well-sanded drive, the same sealant can hold a colour-rich finish for three to five years before it needs a freshen up.

The good news is that preparation is mostly elbow grease rather than specialist skill. It is also the part of the job most people rush, usually because the forecast looks kind and they want to get the sealant down. Hold your nerve. Work through these stages in order and the sealing itself becomes the easy afternoon.

Pick the Right Weather Window in the First Place

Sealing is a fair-weather job, and in the UK that means being patient about it.

• Temperature: aim for a minimum of 10°C and a maximum around 25°C. Below this, most sealants cure slowly or not at all.
• Rain: you need a clear, dry forecast for at least 24 to 48 hours after application. Cheek the radar, not just the icon.
• Drying time before: the paving itself must be bone dry. Paving laid or washed in the last few days will still be holding moisture deep in the sub-base.
• Season: late spring and early autumn are ideal. Midsummer sun can make the sealant flash off too quickly and leave lap marks; frosty months are a write-off.

If your paving sits under trees or beside a north-facing wall that rarely dries, budget an extra few dry days before you go near it with a sealant tin.

Clear Weeds, Moss and Algae First

Vegetation is the enemy of a uniform finish. Moss and algae hold moisture against the block face, which is exactly what you do not want under a film-forming sealant.

• Pull or scrape out larger weeds by hand, working along the joints with a thin scraper or an old screwdriver.
• Treat moss, algae and lichen with a suitable paving cleaner or weedkiller, then leave it the full dwell time on the label. Rushing this stage means dead growth stays stuck to the surface.
• Brush the whole area with a stiff yard broom once the growth has died back and blackened.
• Before washing, clear the drainage gullies and cover any nearby drains so you are not sending silt and debris into the surface water system.

Do not skip the treatment and rely on pressure washing alone. A jet washer will strip the tops off moss but leaves the roots and spores behind, and they will push back through the sealant within a season.

Give the Blocks a Proper Deep Clean

Once the growth is dead, it is time to wash. A pressure washer is fine, but technique matters more than power.

• Use a fan nozzle at roughly 30 to 40 degrees to the surface, not a concentrated jet, and keep the lance moving.
• Work in sections of a few square metres and rinse as you go. Do not let dirty water dry back onto the paving.
• Hold the lance away from the joint sand — too close and you will blow the joints out, creating extra work later.
• For oil, rust or fertiliser stains, apply a dedicated paving cleaner, agitate with a stiff brush and rinse thoroughly.
• Rinse the whole area down with clean water at the end to remove any residue of cleaning chemicals, which can interfere with adhesion.

If white, chalky patches appear as the paving dries, that is efflorescence — naturally occurring salts pushed to the surface. Brush it off once dry; sealing over it just traps it in.

Repair Sunken Blocks, Loose Edges and Drainage Before You Seal

Now is the moment to fix anything that is moving, because sealant will not hold a wobbly block in place — it will simply crack along the joint.

• Lift any rocking or sunken blocks, top up the bedding layer with sharp sand, and re-lay them level with a rubber mallet.
• Check edge restraints and kerbs. If they have shifted, re-bed and re-point them; the whole surface relies on that perimeter being solid.
• Look at falls and ponding. If water sits in a dip after rain, sealant will make it worse by reducing the paving's natural permeability.
• Fill any gaps at the edges of the drive, around manhole covers and against walls, so water cannot track underneath.

Leave any re-laid blocks to settle and dry for a few days before continuing.

Let It Dry Completely, Then Re-Sand the Joints

Drying is the stage people underestimate most. Moisture trapped under a sealant will show as cloudy white patches, and it can lift the film entirely once the sun hits it.

Allow at least 24 to 48 hours of dry weather after washing — longer in cool, humid or shaded conditions. If you can press a dry cloth into a joint and feel any dampness, wait.

Then re-sand. Sweep kiln-dried silica sand or a quality polymeric jointing sand into the joints, working diagonally across the paving and topping up until the joints are filled to about 3mm below the block surface. Compact it gently with a vibrating plate fitted with a rubber pad, then sweep off the surplus.

Give the sanded surface a final dry brush and a quick check for dust, footprints or stray leaves before you open the sealant. A clean, dry, weed-free, well-sanded drive is exactly the surface a sealant wants — and in British weather, that preparation is what buys you the finish that lasts.