To repaint a cast-iron bathtub, cut through any existing caulking between the wall and the tub with a razor knife, and clean the tub thoroughly with a household cleaner. Scrape the tub with a razor, and wipe it down with a cloth that is dipped in denatured alcohol. Fill holes or chips with auto-body putty, and sand the putty until it is smooth. Apply enamel bonding agent to the tub, followed by three coats of enamel paint.
After scraping the tub with a razor, rinse the tub with clean water, and completely dry the tub before applying the denatured alcohol. To sand the tub after applying putty, allow the putty to dry, and sand it with 36-grit sandpaper, followed with 80-grit sandpaper. Use glazing putty to fill very minor holes or imperfections.
Apply the enamel bonding agent to the tub with a clean cloth according to the manufacturer's instructions, and apply the enamel paint to the tub using a paint sprayer. Allow the bonding agent and paint to dry completely between coats. Sand the tub lightly with 1,000-grit sandpaper when the final coat of paint is dry, and buff the tub with a hand or power buffer to achieve a shiny finish.