Concrete window sills are more common on older homes than newer ones. Many of these homes are historical and their owners work hard to keep them in good condition. You may know quite a bit about household repairs, but even knowledgeable homeowners may be confused when it comes to repairing something as unique as a concrete window sill. There are several types of damage that may occur to your concrete seal, and each must be repaired the correct way to maintain the integrity of the window sill and the safety of the window itself.
Rub the surface of any and all damaged areas with a stiff brush. Remove as much concrete debris as possible.
Sand any rough concrete areas with rough-grit sandpaper. Make the concrete as even as possible.
Rinse the concrete window sill well to wash away all of the loosened crumbling concrete and any sanding dust. Wait two days for the concrete sill to dry inside and out.
Horizontal Surface Repairs
Fix any small or medium cracks in your concrete window sill with caulk. Fit the narrow tip of a bottle of silicone latex caulk into each crack and overfill it by about ¼ inch. Spread and flatten the caulk with a small putty knife; scrape away any mess with the edge of the knife.
Fill pitted areas or recreate corners and edges with prepared concrete patch. Push the soft patch product into pits with the corner of a trowel or use the trowel to build up the concrete patch on the edges or corners. Use the flat face of the trowel to smooth and shape the patches.
Resurface stained or cracked concrete window sills with new concrete. Use a paintbrush to apply concrete bonding agent to the entire window sill. Apply a 3/8-inch-thick layer of concrete all over the sill with a trowel, following any contours to mimic its original shape.
Let repairs dry for one to two days and cure new concrete for approximately five days.
Vertical Surface Repairs
Use epoxy, which will not shift, to make vertical crack repairs. Insert the injection ports included in your kit along the crack, pushing them in as far as possible. Place one port at each end of the crack and space the others ½ inch to an inch apart.
Use a paintbrush to coat the damaged area inside and out with the epoxy adhesive. Be sure to get the adhesive as close as possible to the ports but not on top of them. Wait until the adhesive becomes sticky—about three minutes.
Begin filling the injection ports with epoxy, working from the lowest port to the highest. Stop filling the current port when the epoxy is visible in the hole of the one above it. Cover each port with its corresponding cap and let the epoxy dry for two days.
Knock the injection ports at a 45-degree angle with a hammer. Pull each port out of the crack. Use a putty knife or paint scraper to remove any excess epoxy on the window sill.