Things You'll Need
20mm plastic (if needed)
You can cement over a brick chimney easily. The hard part is getting the cement to stick. Most skim coats of concrete will crack and peel off within a few months if you do not know how to properly apply the concrete and care for it while it is curing. The porous nature of the clay brick and the porous concrete skim coat need a little extra help to stay together forever.
Fill a plant mister with water and mist the surface of the brick. You don't want to saturate the brick with water, but you do want it to have a layer of moisture on the surface.
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Mix portland cement (cement with no aggregate) in a bucket with water in at least a 3 to 1 ratio until it has a batter-like consistency.
Scoop your concrete onto your trowel and trowel the concrete onto the brick surface. Smooth the concrete over the bricks in arc-like sweeps, holding the edge of the trowel at a 45-degree angle, to make sure the coating is consistent.
Cover the entire surface of the brick chimney with concrete, making sure you are misting the brick ahead of applying the concrete so none of it is trowelled onto dry brick.
Smooth the concrete skim coat with a magnesium float. Make sure you dip and wipe the float clean in water often as you go, to prevent any drying concrete from collecting on the float face and ruining the surface you are smoothing.
Continue to mist the entire surface every 20 minutes until the concrete has completely dried and cured. Misting the surface will significantly slow the drying time, which is what you want to ensure that the bond between the brick and concrete is strong.
If the chimney is in a dry area, cover the concrete with 20mm plastic after misting it, to make sure it does not dry too fast. This heavy plastic will not damage the smoothness of your float finish.
While in many areas applying a concrete skim coat to a brick fireplace as it is exposed within a house is fine, many areas have specific building code regulations about whether or not it may be applied to the fireplace on the exterior of the building, particularly around the cap. Check your local building codes to make sure before you begin, to avoid any unexpected fines.