You can paint your cast iron bathtub with special paint, so that it looks like new once more.
If there are cracks or the paint inside your bathtub is peeling, scrub or peel it off entirely. If there is rust, you or a professional will have to sandblast it off.
Type of Paint
Oil-based enamel or latex paint made for bathtubs can be used for painting the inside a cast iron tub.
Enamel paints are long lasting and will work with cast iron bathtubs, even after many uses.
Prep your cast iron tub with rust resistant primer prior to painting so that your paint will hold better.
Keep your cast iron tub clean with a few drops of mild dish detergent and a gallon of hot water. Remove soap scum with baking soda and warm water. Do not use abrasive materials to scrub the tub.