Sealing a masonry window sill

Question

I have concrete/masonry external window sills. I have repeatedly treated the sills with a recommended white sealer however rain drips from the gutters keep eroding the edge of the sills. How do I deal with this?

Chris · Accepted Answer

1. Redirect the water

Gutter overflows / drip edge: Check that your gutters aren’t overflowing or leaking at the downspout junctions. If rainwater is constantly dripping on the sill, you’ll need to extend the downpipe or add a gutter extension.

Splash guards / diverters: Some people fit a small metal or plastic drip edge at the bottom of the gutter or sill to throw water away from the sill.

2. Improve the sill’s profile

Add a drip groove: A shallow groove along the bottom edge of the sill (if you can grind/cut it) prevents water from running straight over the edge — it drips off instead of eroding the surface.

Slight slope: Ensure the sill is slightly sloped away from the wall (usually 5–10°). This lets water run off quickly instead of pooling along the edge.

3. Protective coatings

Since standard sealers aren’t enough:

Use a high-quality masonry sealer with abrasion and water resistance: Look for penetrating silane/siloxane water repellents, which soak into the concrete rather than just sitting on the surface.

Brands: Cromadex, Ronseal Silane/Siloxane Masonry Sealers (UK), or similar.

These can reduce water absorption and help prevent erosion.

Topcoat options: For aesthetics, you can use a flexible acrylic masonry paint over the sealer — but make sure the sealer is compatible and breathable.

4. Repair damaged edges

If the sill is already chipped or eroded:

Use a masonry repair mortar / concrete repair putty to rebuild the edge.

Smooth it and, if possible, shape it to include a drip edge or slight overhang.

Once cured, seal it properly with a water-repellent treatment.

5. Long-term approach

The key is reducing water impact first (gutters, slope, drip grooves), then repairing and sealing. Sealers alone won’t stop erosion if rainwater constantly runs over the edge.

For heavily exposed sills, some homeowners even fit thin stone capping (e.g., natural stone over the sill) to give extra durability against rain erosion.

Tony · Answer

Best Method for Sealing a Masonry Window Sill

1. Prep the Surface Thoroughly

•  Clean off dirt, algae, moss, or mould using a fungicidal wash.
•  Sand down flaking paint or rough patches with coarse sandpaper.
•  Scrape off loose material and dust the surface clean.

2. Repair Any Damage

•  Use a cement-based or masonry filler like Toupret Rock Solid Filler for cracks or chips. Avoid overfilling—it’s tough to sand down A once cured.

3. Prime the Surface

•  If the sill is chalky or previously painted, apply a stabilising primer like Zinsser Peel Stop or Dulux Weathershield Stabilising Primer. This helps seal porous areas and prevents future B A peeling.

4. Seal with Masonry Paint

Christian · Answer

Fix the gathering first seal the masonry sill and repaint!

Steve · Answer

Hi Charlie

I would look at using zinsser gaurdz primer then painting with plilote masonry paint in desired colour.

And maybe looking at fixing the guttering drips also

Nicholas · Answer

If you sand down, prep and apply alkaline resisting primer, then 2coats of masonry paint applied

Michael · Answer

You will be best to get all your gutters check then sometimes house drop like sink due to foundation but only on old house meaning your sills could be unlevel so its making the water run into the corners

Tom · Answer

Hi there, 
You could try cement based sealer in this case, you could buy it in Toolstation.

Lukas · Answer

Stop the drip at source, repair the sill edge with a suitable mortar/epoxy, and then paint with a tough exterior masonry paint (not just sealer). If you want belt-and-braces protection, add a drip bead or cover strip to the sill edge.

Fazal · Answer

Fix the gutters – clear blockages, check joints, and make sure water flows to the downpipe. Add an extension or deflector if water drips onto the sill.

Clean the sill – remove loose bits, dirt, and moss; let it dry.

Repair damage – fill chips and worn edges with exterior masonry repair mortar; let it cure.

Reseal – apply a silane/siloxane water repellent for long-lasting protection.

Protect the edge – fit a drip edge or thin cap (aluminium, stone, or stainless steel) to guide water away.

Maintain – check gutters and sill regularly; reapply sealer as needed.

✅ Summary: Stop the gutter drip first, then repair and seal the sill, and add a drip edge for long-term durability.

Tomasz · Answer

check out and fixed guttering 
zinsser Guardz it’s the one i recommend the most

Sealing a masonry window sill

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