Seal a concrete block wall by cleaning and patching the surface and then applying a waterproofing paint or sealant designed specifically for masonry. For added protection against moisture and weathering, opt for a sealant that contains antibacterial and antifungal agents.
To prepare a concrete or cinder block wall for sealing, first thoroughly clean the wall using either a stiff wire brush or a shop vacuum to remove any dust or debris. If the wall is wet, allow it to dry completely before proceeding. Next, patch any holes or cracks using an expanding hydraulic cement, which takes approximately 24 hours to set and dry. If a portion of the wall that needs to be sealed is in an enclosed space, ventilate the space by opening all surrounding doors and windows, and wear protective gear, such as rubber gloves, safety goggles and a respirator. Use drop cloths to protect any flooring around the wall.
Apply the sealant or paint using either a paintbrush or roller designed for masonry or a sprayer. Start applying the sealant in the corners and edges of the wall using a brush, and work inward approximately 6 inches before applying the sealant to the rest of the wall using the roller or sprayer. Masonry paints and sealants dry quickly, making drips or streaks disappear. This means that you can apply the sealant in any method that is comfortable for you. Follow the instructions on the sealing product to determine the number of coats and the drying time in between applications.