Concrete cinder blocks are common in all manner of construction projects, ranging from landscape retaining walls to out-buildings such as tool sheds and even commercial and residential buildings. However tough the actual blocks might be, they are held together by mortar and mortar has a tendency to crack and eventually disintegrate due to heat, cold and moisture. The process of repairing the mortar in a cinder block wall is called "repointing."
Remove badly crumbled mortar by pulling it out with your fingers or scraping it out with a wire brush. Remove cracked mortar by chipping it out with a hammer and cold chisel.
Make a batch of mortar sufficient for a 30-minute work session, adding the proportion of water directed by the manufacturer. Pour the mortar into the mortar bag.
Place the nozzle of the mortar bag just inside the empty joint between blocks. Put mortar into the joint by squeezing the mortar bag and pushing mortar out the nozzle. Do not attempt to fill the entire joint with mortar but instead draw a thick bead of mortar, as you would with a caulking gun.
Place your trowel over the joint and scrape off any extra mortar. Flick the extra mortar off the trowel and into the mortar bag. Move on to the next empty joint.