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Chisel out two or three small chunks of existing mortar for analysis. In a well-ventilated area -- preferably outside -- and wearing solvent gloves and safety glasses, pour a small amount of Muriatic acid into as many glass jars as you have mortar samples.
Note: Muriatic acid is a dangerous chemical. Please read all safety precautions before using. Be sure to avoid use around children, pets and plants, and consult local authorities for proper disposal method. If you feel uncomfortable, please send mortar to lab for analysis.
Gently, place one chunk of mortar in each jar of Muriatic acid. Do not splash. When the samples stop reacting to the acid, drain off the acid. The remnants will indicate the original sand color that was used.
Use different sand colors to mix one part lime with three parts sand to a thick frosting consistency. Try to match the batches to the original mortar color, keeping in mind that the color when change slightly when dry.
Wearing a balaclava (a knit hood that keeps dust from your hair and face) and a respirator, begin grinding the existing mortar away with a 4" angle grinder, to about 1/4" depth. Blow away excess dust after grinding.
Next, wet the brick to prevent the brick from "wicking" water from the new mortar, and keep the mortar from cracking after it dries.
With the proper color selected, prepare mortar in the amount needed (Image 1) to complete the repointing project.
Add a small amount of mortar to the end of the trowel. Position the trowel against the edge of the brick, then use the pointing tool to work mortar into the crevice (Image 2). Work toward the side -- never pull the pointing tool forward. (Use a smaller trowel for vertical joints, but apply mortar in the same way.)
After the mortar has set up -- about one to three hours -- use a steel brush to remove excess mortar from the face of the brick. Take care not to hit the newly repointed joints.
To achieve the same texture as the original mortar, use a damp cloth to lightly tamp over the mortar joints.
Approximately one week later, use a Muriatic acid and water solution to wash over the face of the brick to remove any residual haze from the mortar.
Here are a few tips to help ensure the longevity of your newly repointed mortar:
If properly maintained, repointing can last 25 to 50 years before it needs to be repeated.