Inspect the walls for any signs of mold. Mix six parts of water with one part of bleach and scrub the walls with a brush. Let dry.
Clean any loose debris from the walls with a wire brush. Fill any cracks with quick setting mortar. Let dry.
Apply a stabilizing primer to the walls. This will help to seal the wall, making the finish coats adhere all the better. Let dry.
Apply finish paint to the rendered wall with a paint roller. For the parts you can't reach with the regular handle, uses a longer, extension handle.