Waterproof coatings are more effective when applied to the outer faces of the walls. However, if the water pressure from leakage is small, cement-mortar coatings may be applied successfully to the inside face.
Active leaks through cracks may usually be sealed by plugging them with a cement mortar containing an accelerator to hasten hardening.
The masonry around the crack should be cut away to a depth and width of 1 or 2 inches. A stiff putty should then be prepared from high-early-strength Portland cement, sand, and calcium chloride up to 8 percent by weight of the cement.
The mortar mix may be 1 part cement to 2 or 3 parts sand. The putty should be pressed into the opening and held by a small board or form until it has hardened.