A crumbling, flaking, or cracked concrete window sill is more than an eyesore. If left unchecked, water seeps into the cracks, freezes, expands, and causes further spalling (flaking). Eventually, this can lead to water damage inside your walls or even structural failure of the sill itself.
Using your trowel, press the patching material firmly into the hole. to force it into all voids and eliminate air pockets. Overfill slightly, about 1/8 inch above the surrounding surface, to allow for sanding or shrinking. Step 6: Shape and Finish the Surface Once the patch is in place (typically 15-20 minutes after application, depending on temperature), use your finishing float or a damp sponge to shape the surface. Recreate the original slope (usually about 5-10 degrees) so water runs away from the window. Smooth the patch flush with the existing concrete edges. how to repair crumbling concrete window sill