Add 2 tablespoons of borax to a small bowl.
Stir in enough fresh or concentrated lemon juice to make a thick paste.
Wet a nylon scrubbing pad with lemon juice and dip it in the paste.
Scrub the rust stains in the tub until they disappear.
Rinse the tub with water, and dry the surface of the tub with a rag.
Pour white vinegar into a spray bottle.
Mist the rust stains liberally.
Sprinkle salt over the rust stains.
Wet a rag with white vinegar and scrub the salt into the rusty stains until they are no longer visible.
Rinse the bathtub with water, and dry the surface with a rag.
Cut a large onion in half with a knife.
Rub the rust stains with the cut side of the onion for five minutes to spread the onion juice over the stain.
Wipe the rust stains out of the tub with a rag. Repeat if necessary to remove any remaining rust stains.