To remove a rust stain from fabric, rub the stain with a paste of hot water, cream of tartar and salt, and launder the fabric with a regular detergent in a washing machine according to the manufacturer's instructions. Air dry the fabric to complete the process.
Prepare a paste of equal parts tartar cream, hot water and salt. Apply the paste to the stain, rub the fabric to loosen the stain, and then rinse the fabric in a washing machine. For a smaller stain, cut a lemon in half, squeeze the juice onto the stained area, and wash the fabric as normal. The acidic property of the lemon breaks down rust particles.
To remove a stain from delicate fabric, soak the fabric in boiling water, apply lemon juice to the stain, and rinse the fabric. Repeat the process until the stain disappears, taking care not to discolor or destroy the fabric. If the stain persists, add lemon juice to the prepared paste, and rub the affected spot. Let the fabric sit for half an hour, and then rinse and air dry it.
For a stubborn stain, dampen a cloth with industrial acid, and use the cloth to rub away the stain. This method works for hardwearing fabrics, including denim.