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- 1). Clean out the holes by vacuuming them with a shop vacuum to remove any loose grit. Take a good look at the hole with a flashlight if it reaches beyond the surface of the block. Most small holes don’t reach the center holes of the cinder blocks; but if you have a larger hole, you will need to do a double stage repair.
- 2). Force the tube of a construction spray foam canister into the openings of any holes that reach the empty cavity in the cinder block and spray for a few seconds. If the tube does not fit into the hole, drill an opening with a masonry bit. Only fill one-third of the cavity so the foam has room to expand. If extra foam extrudes from the hole, let it dry for about 30 minutes then scrape out the hole with a sharp object.
- 3). Widen the opening of the hole with a hammer and chisel so that the foam expands just slightly inside the block. As the concrete dries, you want it to push against the opening of the hole without dislodging the construction spray foam. Vacuum out any loose sand or debris.
- 4). Mix up equal parts of sand and mortar and push it into the hole with a putty knife. Smooth the repair mortar even with the surface of the wall. Allow it to dry for 24 hours before sealing or waterproofing.
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