Painting your cast iron bathtub can add a needed pop of color to your bathroom. While the interior of a cast iron bathtub should always be painted by a professional, the outside of the cast iron bathtub can easily be painted any color that fits with the rest of your bathroom. Also, painting the outside of a cast iron bathtub can also ensure that any rust is inhibited before it starts.
Traditionally, black has been a popular color for the underneath exterior of a cast iron bathtub. However, any color can be used, provided you can find the color you need in an oil based enamel. Keep in mind that oil based paints can be incredibly difficult to find: selection often varies from state to state, depending on applicable environmental laws.
Step 1: Remove any existing paint and rust with 150 grit sandpaper, making sure the entire exterior surface of the cast iron bathtub is lightly sanded.
Step 2: Apply a coat of rust resistant primer to the outside surface of the cast iron bathtub. Look for an oil based product for this step when possible; it may be difficult to find in some states. Let the primer dry before proceeding.
Step 4: Using a piece of fine grit, wet/dry sandpaper, remove any obvious drip marks or brush strokes from the exterior surface of the cast iron bathtub. Be sure to dampen the areas that need to be sanded for best results. Wait until the entire areas is dry before proceeding.
Step 3: Apply a second coat of the primer with a paintbrush and allow it to dry before continuing. Remove any drip marks with sandpaper, but be sure not to sand so deeply that the primer is completely removed from the outside of the cast iron tub.
Step 4: Apply a coat of enamel, oil based paint with a paintbrush. Allow the first coat to dry, then sand away any rough areas on the exterior of the tub.
Step 5: Apply a second coat of the enamel. Once dry, evaluate the finish. If you like the color and finish, then your job is complete. Otherwise, plan on wet sanding and adding another coat of enamel to the outside surface of the cast iron bathtub as necessary.