Clean the walls. Use a wire scrub brush to clean off any dust or debris. This will help your concrete sealer adhere better. Use a putty knife to pick away at any paint chips or other materials stuck to the walls. After cleaning, rinse the walls thoroughly with water. Allow this to dry.
Wash the walls with muriatic acid. This will help remove any other deposits, such as salt, not removed by conventional cleaners. Mix the muriatic acid with water in a solution of 1 part muratic acid to 10 parts water. Be sure to wear goggles, gloves and a safety shirt before applying. Use the wire brush to scrub the walls thoroughly with the acid solution.
Fill any cracks with concrete. First, widen the cracks. You can do this with your putty knife. The inside of the crack needs to be bigger than the outside for the concrete to adhere properly. Wet the area around the cracks with water from a garden hose or bucket. Mix the concrete in a bucket until it has the consistency of toothpaste. Apply it with a concrete trowel. Make sure the concrete reaches all the way to the back of the crack. Allow the patch to dry for a few hours. Take a piece of plastic, cut it to fit over the crack, and secure it to the wall over the crack with duct tape. Once a day for the next five days, lift up the plastic and wet the concrete. The concrete should be cured by the fifth day.
Read the manufacturer's directions for your waterproof sealer. There are two major types of sealers available for waterproofing concrete walls: liquid compounds and concrete. Before you apply a liquid compound, the walls must be dry. Cement compounds, on the other hand, require that the walls be damp before application. Follow the instructions that came with your product.
Apply the wall compound. Make sure that you get all the cracks, joints and other nooks and crannies covered. You can apply a second coat after the first one is dry.