Cinder blocks walls are frequently encountered in out-buildings such as garages and storage sheds, as well as in basements. Although the concrete cinder block itself is robust and can bear enormous weight, cinder block walls are erected using regular mortar mix, which can crack and crumble. Sometimes the damage to the mortar in a cinder block wall is minor and can be fixed with a little touching up, but in other cases the damaged joints should be repointed.
Mend small cracks in the mortar with a tube of acrylic mortar repair compound. Put a bead of the compound into the crack and spread the top of the bead over the surface of the joint with a putty knife.
Chip away the mortar for large cracks or crumbling areas using a hammer and a cold chisel. Remove mortar from the joint to at least 1 inch on both sides of the damaged area. Be careful not to chip or crack the cinder blocks.
Scrape out any remaining bits of mortar from the joints to be worked on with a wire brush.
Mix a batch of mortar in a bucket or other suitable container sufficient for 30 minutes worth of work. Pour this mortar into a mortar bag.
Squeeze the mortar bag to push mortar through the bag's nozzle and fill the empty joints in the cinder block wall. The mortar bag works a lot like a cake decorator, so control the flow of mortar by adjusting your squeezing pressure.
Scrape away extra mortar from the joint and then shape the mortar to match that of the original mortar in the surrounding joints.