Vacuum the walls with the bristle attachment and a long handle to remove any crumbling parts on masonry walls and dust.
Wash any mold spots on the walls using a scrub brush and a solution of equal parts bleach and water. Allow it to dry thoroughly before proceeding.
Cover the flooring with a drop cloth and tape around window and door frames using painter's tape.
Stir the Polycell SmoothOver and dip the SmoothOver sponge roller into the container so the roller is completely submerged.
Roll the SmoothOver product onto the wall, working in overlapping sections. It will have a textured appearance when you apply it. Reload the sponge roller as needed.
Scrape the wall with the SmartSmoother to remove excess product and give the wall a flat surface. Do this every 3 square feet so the product doesn't dry while you are working. This is key to getting a smooth surface without any ridges.
Leave the wall to dry for 12 hours.
Sand any rough spots with 100-grit sandpaper and wipe up the sanding dust with a tack cloth.
Paint the wall as desired.