Manufacturers bake a porcelain finish over cast-iron tubs or apply an enamel finish to protect the metal from rusting. Through use and age, the coating can crack, chip or wear, making the bathtub look old and worn. Restoring and reviving colour to a bathtub is both cost-efficient and aesthetically pleasing. Repainting a cast-iron bathtub is much less expensive than buying a new one.
Turn off the main water supply to the tub. Secure a plastic bag around the faucet and shower head.
Remove the caulking from around the tub, if applicable. Cut through the caulking with a utility knife and pull caulking away from the tub.
Sand the surface of the bathtub with 120-grit sandpaper loaded into a palm sander. Sand surfaces that you cannot reach with the palm sander by hand. Sand rusted surfaces until you see the metal.
Vacuum the tub to remove dust and debris. Wipe the bathtub with a tack cloth to pick up bits of rust and dust.
Apply light-tack masking tape to faucet handles and around the perimeter of the bathtub where it meets with the wall. Lay a tarp over other exposed surface in the room. Tape light plastic over walls to protect them from paint splatter.
Dip a lint-free rag into a rust-inhibiting, epoxy-based bonding agent. Spread the bonding agent over the entire surface of the bathtub in a light and even coat. Allow the epoxy-bonding agent to set according to the manufacturer's directions, generally five to 10 minutes.
Pour an epoxy-based primer into a paint sprayer. Apply an even coating of primer to the entire surface. Work from side-to-side or back-and-forth. Overlap each pass by 1/2 inch onto each other. Allow the primer to dry completely, usually two to four hours depending on temperature, humidity and the primer.
Spray on a second coat of primer. If you moved the sprayer back-and-forth on the first pass, move it side-to-side. If the first pass was side-to-side, spray the primer on using a back-and-forth motion. Allow the primer to dry for two to four hours.
Pour an epoxy-based paint into the paint sprayer. Spray two coats of paint onto the surface; allow each coat to dry for two to four hours between sprays. Alternate applications between back-and-forth and side-to-side.
Wet a piece of 400-grit sandpaper and lightly sand the paint finish. Wipe the tub down with a tack cloth.
Apply a third coat of paint to the tub surface and allow to dry fully.
Pour a clear epoxy-based sealer into the paint sprayer. Apply two coats of sealer to the surface. Let each coat dry completely between sprays, typically two to four hours.
Work in a well-ventilated room. Wear safety goggles, respirator and gloves when applying epoxy-based primer, paints and sealers.