If you have puddles or pools of water from a leak, remove anything that may become damaged and clean up the water with a mop or towels. Remove any mold or mildew that might have formed (see project Remove Mold from Walls).
Next, remove any loose or broken mortar, dirt, dust or other debris around the leak, using a broom or stiff brush. Clean the surface thoroughly. Waterproofing works best when applied to a clean, bare masonry surface.
To remove old paint, use a wire brush or sandblast the surface. Use muriatic acid to get rid of efflorescence — white, crystal-like salt deposits that form on the wall from moisture. Any efflorescence left on the wall will prevent a sealant from sticking to the wall. Any holes should be patched with hydraulic cement.
Muriatic acid is highly corrosive and can be harmful to your skin. Wear protective clothing, safety goggles and protective gloves when working with muriatic acid. The fumes from muriatic acid can also be harmful. Be sure you're working in an area with proper ventilation. In a basement, you may need an exhaust fan to help ventilate the area. Muriatic acid can also damage other surfaces. Take care when applying the muriatic acid to walls.