Before You Begin
Set up a ladder so it is safe and within reach of the loose brick mortar of the chimney. Wear heavy gloves and safety glasses for protection. Make sure that the fireplace damper inside the house is closed to prevent any loosened soot from falling down into the hearth.
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Step 1 : Remove loose mortar from the joints
Use a chisel holding it at an angle to the joint and strike it with a hammer to knock out the crumbling joint to a depth of about 1/2-inch. Chip out both the vertical and horizontal joints.
Step 2 : Clean out the joints
Use an old paint brush to remove loose mortar and dirt from the joint between the bricks. Brush it out several times until the joint is clean and dry.
Step 3 : Dampen the joints with water
Moisten the joints with a spray bottle of water to dampen them.
Step 4 : Use a trowel to apply the material
Prepare as much concrete or mortar mix that can be used in one hour according to the manufacturer's direction. Then apply it with a trowel working it into the vertical joints first, then the horizontal joints to provide a permanent seal.
Step 5 : Strike the joints
Use a jointer to strike the joints to a slightly concave profile. After the mortar hardens, brush off any loose mortar.