Clean the wall thoroughly, using a power wash system or a bucket of warm water and a scrubbing brush. The wall needs to be both clean and clear of debris, such as dirt and mortar. Allow the wall to dry thoroughly overnight.
Pour waterproof primer, which will act as a sealant, into the paint tray. Roll the paint roller through the primer, and squeeze off the excess on the side of the paint tray. Apply even coats of primer to the wall, using the paint roller. Wait an hour for the primer to soak in, and then apply a second coat evenly with the paint roller. Wait 24 hours for the primer to soak in and dry completely.
Examine the wall for cracks and holes. Fill these with liquid rubber. Smooth the surface around the repair, using a putty knife. Follow the instructions provided with the liquid rubber. Apply an even coat of waterproof sealer to the wall, using the paint roller. Fill in any uneven areas with the paintbrush and more sealer.
Wait two hours for the sealer to dry and set properly. Apply another coat of sealer evenly to the wall, using the paint roller. Allow this coat to dry, and then apply another coat. Again, go over uneven areas with a paintbrush to ensure that the sealer has worked into all the nooks. Allow 24 hours for the sealer to dry thoroughly. Apply a coat of concrete finisher evenly to the wall, using the paint roller.