Rust stains in a bathtub can be caused a number of other factors, such as leaving a can of shaving cream on the corner of the tub. When metal becomes wet, starts to rust, and then comes in contact with a porcelain bathtub, it can leave an unsightly stain. Hard water can also cause a bathtub to become rusty over time. Luckily, there are several different methods you can use to clean rust stains from a bathtub, from common kitchen items to commercial cleaners.
Cut a lemon in half using a sharp kitchen knife. Rub the rust stains lightly with half of the lemon, removing as much rust as possible. For very light stains, the lemon may remove most or all of the rust.
Pour hydrogen peroxide liberally over the rust stains. Sprinkle with cream of tartar, creating a thin layer over the peroxide. Allow the mixture to sit for at least 30 minutes, and then scrub clean with a nylon scouring pad. Very stubborn rust stains may require several applications before the bathtub is completely clean.
Combine two parts borax powder with one part lemon juice to make a thick paste. Rub onto the rust stains and scrub clean with a nylon scouring pad after about 30 minutes. Repeat, as necessary, to remove all the rust.
Soak a few layers of paper towels in white vinegar. Cover the rust stains with the towels, and allow them to sit for at least three hours. If the stain is gone after this time, remove the paper towels and rinse with cold water. If the stain is still there, replace the soaked towels and allow them to sit overnight.
Use a commercial rust stain remover as a last resort. Use it according to the manufacturer's instructions, and make sure to check the label for safety precautions.