Refinishing a cast iron bathtub involves cleaning and repairing the tub surface, covering the surface with enamel bonding agent, spraying the surface with acrylic urethane enamel paint, and smoothing and buffing the surface. Protect yourself from toxic gases with a respirator before painting the surface.
Start by scraping the tub's edge of caulk with a utility knife and detaching the drain cover. To clean the tub, saturate it with a bathroom cleaner, scrub away all dirt and debris from the tub's surface with a brush, and apply clean water to remove the cleaner residue. Using a razor blade, clear the surface of any deposits, including soap scum, and rinse it with clean water again. Bloat the tub with a clean, dry rag. Dampen a clean rag with denatured alcohol, and apply the alcohol to the surface.
To repair the tub surface, first trowel an auto body filler, and apply it over chips and dents on the tub. Allow the filler to dry before smoothing the surface with sandpaper. Even out the surface by applying glazing putty, and allow the putty to dry before smoothing the surface with sandpaper. Use a tack cloth to dust the surface.
To paint the surface, dab an enamel bonding agent on a clean cloth, and paint the agent over the surface with the cloth, adding more agent to the cloth as needed until you exhaust the entire surface. Pour acrylic urethane enamel paint in a paint sprayer, and apply three coats of the paint to the surface, allowing the paint to dry between the coats. Finally, smooth the surface with wet, 1,000-grit sandpaper, and restore the surface's shine by buffing.