A porcelain bathtub can develop rust stains that will mar its beauty as well as attract mold and mildew over time. You can use the abrasive nature of muriatic acid -- a common mineral acid -- to rid your bathtub of rust stains. Muriatic acid is available from your local hardware store and must be treated carefully as it is quite caustic.
Put on a pair of gauntlet-type work gloves and protective goggles. Pour 4 oz. of room-temperature tap water into a measuring cup. Add 4 oz. of muriatic acid to the measuring cup.
Dip an edge of a piece of terry cloth into the mixture in the cup. Place the edge against the bathtub along one side of the rust stain. Press down on the edge as you drag the cloth across the stain. Lift the cloth off the bathtub once you reach the other side of the stain. Repeat this once more.
Sprinkle baking soda on the area where the rust stain was to neutralize the acid.
Dampen a clean cloth with tap water. Sprinkle a few drops of dish washing liquid onto the cloth. Wipe the area where the rust stain was with the cloth.