Using a waterproofing paint on the interior of the foundation walls could be the worst option to do in comparison to doing nothing. Concrete blocks and cinder blocks have hollow cavities inside them. This means these walls are completely hollow. Ground and surface water can make access inside your walls through cracks, mortar joints, or other deficiencies causing your walls to fill with water. The paint seals this moisture and water in and causes your walls to hold stagnate water. Moldy walls will be your immediate symptom, followed by water inevitably making its way in.