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How to repair worn window sills

We have already explored how versatile window sills can be.  With internal sills being constantly in use and exterior ones open to the elements, let’s take a look at how to care, properly, for both internal and external window sills.

Damage to a sill is often the result of a fault in the construction of the window, basic design or former repairs. Window sills don’t just need to look good, they need to perform their function optimally. Whether that be showcasing porcelain figurines in a dining room or propelling rainwater from a supporting wall,the condition of the window sill is crucial.

Spotting the signs of trouble

When water is propelled continuously against a window, the downslide can often penetrate into the joints of the window sill, which over time can result in rotten wood or crumbling concrete. When paint cracks the water begins to penetrate into the underneath surface and gets trapped. Pretty soon rot fungi spores appear and they begin to eat the wood of the sill which left untreated could spread to the window frame itself. With concrete and slate sills, the exposure to the variations in temperature can cause cracks and crumbling, water penetration can also weaken the joints.

To avoid this, check regularly for:

  • Cracked or worn paintwork.
  • Puddles of water collecting on the window sill.
  • Worn or loose joints.
  • Cracked concrete.

Repairing a woden sill

Before taking on a repair job, it’s worth noting that the experts’ rule of thumb is, according to Ultimate Handyman, “if the whole thing is rotten over 30% then it needs replacing.” If the sill is worth saving then to repair it you will need the following equipment –

  • Chisel or old, wide screw driver.
  • Wood filler (high performance type).
  • Filler knife (plastic or metal).
  • Small piece of wood (to mix the filler)
  • Wire brush.
  • Various grades of sandpaper.
  • Primer.
  • Undercoat.
  • Top coat.


Sand, scrape or wire brush off the flaking paint, then gouge out all the soft, rotten wood with the screwdriver or chisel. Ensure the area is clean from dust before packing the area with the mixed filler. Note that the filler will harden very quickly, so ensure you have smoothed out and covered the whole area, leaving no air holes or gaps. After about 30mins the filler should be dried, but test first with the screwdriver.

Begin sanding with a coarse grade, then swap to a medium, followed by a fine paper when the filled area begins to blend in with the wooden sill surface. Always apply a primer before painting, as the primer will expose any lumps or bumps that may require further sanding. Paint with undercoat and a couple of layers of top coat to counteract the harsh weather.

Repairing concrete window sills

Check sills for cracks and crumbling. The experts agree that unless the concrete has fallen off, to the degree whereby the inner steel support is visible, repairs are always possible. For this you will need –

  • Wire brush.
  • Soft brush.
  • Multi purpose exterior filler / sand & cement mix.
  • Filler knife or spatula.
  • Masonry, floor or doorstep paint.


1. Rub off flaking paint or loose concrete along the crack with the wire brush and make sure to clean out any moss that may be growing inside; as this may prevent the filler from making a good seal. Remove any dust with a soft brush.

2. Fill in the cracks or broken concrete with either the multi purpose exterior filler or the sand and cement mixture (ensuring it’s a fairly wet solution) using the filler knife or spatula. Make sure that all the cracks are filled in properly. Note that you should not attempt this process when it is raining, as the filler, sand and cement may get diluted and therefore lose some of its strength.

3. Allow to dry for at least 4 days before you paint with either masonry, floor or doorstep paint.


Using a top quality exterior paint, and applying with due care and attention can protect both wood and stone for many years. Being vigilant and checking those window sills regularly for wear and tear will, however, help to avoid major repairs or even costly replacements. In our next post we will look at how to replace a window sill, if the worst does happen.