Rust develops on a bathtub because the surface chips or cracks and exposes the metal. Rust can also develop due to a high iron content in your water supply. Determining the type of rust will determine the removal method. Manufacturers cover cast iron bathtubs or other metals in porcelain or enamel. When damage occurs to the plating and the metal comes in constant contact with water, it oxidises and rusts. In areas where the iron content in water is high, tiny deposits of iron remain on the surface, oxidise and dry into red-brown or yellow-brown stains. To determine the cause of the rust, closely examine the tub and feel for nicks or deep scratches. If you feel or see damage, the underlying metal is the problem. If you do not see damage, the water supply is the problem. Choose the remedy based on the type of rust.
Wet the stains with lemon juice or white vinegar and pour a generous amount of borax over it.
Soak a paper towel in lemon juice or white vinegar. Lay the paper towel over the rusty area for 12 to 24 hours. Re-apply the acidic remedy, if the paper towel dries.
Remove the paper towel and wipe away the borax and the rust stains. Repeat if necessary.
Dry the bathtub thoroughly with a rag or towel. Turn off the water supply to the bathtub in order for the bathtub to remain completely dry during the repair process.
Sand the rusty area with fine-grit sandpaper to remove all traces of rust. If rust covers large areas of the tub, equip a belt sander with fine-grit sandpaper and sand off the rust. Sand until you expose the metal under the rust.
Vacuum the bathtub to remove loosened rust. Pick up fine bits of rust with a tack cloth. A tack cloth is a sticky cloth that grabs tiny bits of debris and holds them on the cloth.
Paint the exposed metal with an epoxy-based primer and allow the primer to dry for two to three hours. For small chips, use an artist's paintbrush. For large surface areas, use a low nap paint roller.
Apply two coats of a high-gloss epoxy-based paint over the primer. Allow each coat to dry for two to three hours between applications. Choose a closely matching colour to blend into the existing finish.
Dry the bathtub after each use to avoid water-based rust. Repair small chips and cracks as soon as they occur to avoid extensive repairs.