To restore a cast iron tub, first remove the rust with a power sander or sandpaper. Apply only light pressure in case the rust made the iron brittle. Once the rust is gone, clean the tub with a rag and liquid cleaner. Allow the tub to dry completely before repairing nicks, scratches and gouges with a trowel and fiberglass putty mixed with a hardening agent. Allow the putty to set for at least three hours.
Once the putty is dry, sand the mended areas until they are flush to the tub with no bumps or dips. Apply enamel bonding agent in a thin layer over the tub to help paint bond to the surface. Allow the agent to set for at least two hours. Use an acrylic urethane enamel paint to paint the tub.
Paint in thin, but uniform coats that cover the surface without allowing the iron to show through. Allow the first coat to dry for about five hours, then apply another coat for an even finish. Alternately, use a paint spray gun to make the paint application easier and more uniform. Perform the bonding coat application and all painting in an area with good ventilation, as both produce noxious fumes.