Before preparing any cement patch, you may need to actually chip away at the hole in your sill to get its edges closer to 90 degrees. The reason for this is that a shallower edge is more likely to let moisture work its way under your patch. A sharper edge will help prevent that.
Next, moisten the substrate that you're patching. Moistening will eliminate dust and particles. It will also ensure that moisture from your patching material won't wick into the substrate too fast, thereby drying it out.
With the substrate moistened, the next step is to put down a slurry coat, which is a wet mixture of Portland cement and water. You want the slurry coat wet enough to be spread, but not so wet that it will run. The slurry coat will help bond the patch material to the sill. Use a paintbrush to apply the slurry, and cover the 90-degree edges with slurry as well. It's also important to apply the patch material before the slurry coat dries.
Next, mix the patching material. In a bucket, dry mix 3-1/2 parts mason sand to 1 part white Portland cement. Stir until the mixture is uniform.
The color of sand that you use in your mix will affect the color of your patch, so try to find sand with a color and texture similar to that of your sill. The odds are good that if you use a local sand in your mix, then it will be the same as what was used in the original cement.
Keep a second bucket with water nearby. Dip a sponge into the water bucket, and then squeeze it out into your dry-mix. This will give you a more controlled method of mixing in the water. You'll want the mix to be fairly dry (it should have a consistency of putty), so that it will better stick in the patch. If it's too wet, then you'll have to wait a long time for it to dry.
Next, start working the mixture into the patch with a margin trowel. Work the mix from the center to the edges, which will prevent moisture from later working its way under the patch. Don't worry about making your patch look perfect, since you will smooth it out later.
Once the patch has been added, you'll want to use a straight edge to smooth out the patch, so that it matches the shape of the sill. Slowly drag the straight edge over the patch toward you, being careful to follow the sill's angles. When you're finished, the patch should be relatively level with the rest of the sill, but be sure to leave a little bit of material left over for the next process, sponging.
Take a stiff sponge and lightly dampen it with water, then run the sponge gently over the patch. As you work, rub the sponge towards you, not away, and work the sponge from the corners back. The goal is to blend the edge between the patch and the sill, while also exposing the grain of the sand to mimic the sill's texture.
If you do happen to work the patch down too much, or any material comes loose, you can reapply the patch over that portion, then use the straight edge and sponge over it again.
The patch will take about 28 to 30 days to cure. You can use a breathable sealer on the sill after the patch has cured, which should prevent additional moisture from getting into the patch since much of restoration is all about repairing moisture damage.
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