Avoid applying mortar in extreme weather conditions because the mortar won’t properly set up.
Chip away cracked and loose mortar using a slim, cold chisel and a hammer; remove the existing material to a depth of approximately half an inch.
Be sure to wear safety goggles to avoid catching a piece of flying mortar in the eye. Use the cold chisel slowly and carefully, to avoid damaging the surrounding brick. Clean up all the loose material and dust using a brush after you finish chiseling.
Prepare your mortar and allow the mix to set for about five minutes.
You can buy mortar premixed, or you can create your own batch using one part masonry cement and three parts fine sand. In either case, add enough water to create a paste — about the consistency of oatmeal. It’s best to keep the mix a touch on the dry side. If it’s too runny, it’ll be weak and will run down the wall, making it difficult to apply.
Brush the joints with fresh water.
Doing so removes any remaining dust and prevents the existing mortar from drawing all the moisture out of the new mortar. Otherwise, the mortar can be difficult to apply and will most likely crack.
Apply the mortar using a pie-shaped trowel called a pointing trowel.
Force the mortar into the vertical joints first, and remove the excess (to align with the existing adjacent mortar) using a brick jointer. The brick jointer helps create a smooth and uniform finish. After all the vertical joints are filled in, tackle the horizontal ones.
A week or two later, after the mortar has had the opportunity to set up, apply a coat of high-quality acrylic or silicone masonry sealer to the entire surface (brick, block, and mortar).
The sealer prevents water damage, which is especially important if you live in an area that gets particularly cold. Unsealed brick, block, and mortar absorb water that freezes in cold weather. The water turns to ice and causes the material to expand and crack. Periodic sealing prevents this situation from occurring.