Synthetic materials, such as acrylic and polyester, should be cleaned directly with a mild detergent. Use a commercial laundry bar or cleansing powder (such as borax). Work the detergent in from the underside of the stain so as not to rub the stain in deeper, and use plenty of cold, running water.
To clean rust from flannel and cotton, apply lemon juice to the stain and pat cream of tartar on top of the affected area. This should draw the stain out. Rinse with fresh, cold water before the lemon juice dries. If there is still some discoloration, try applying lemon juice and exposing it to sunlight for 20 to 30 minutes.
When dealing with more delicate items such as silk, leather and suede, it is best that you bring the item to a professional cleaner. These materials are far too temperamental to attempt removal without some level of expertise.
To remove iron rust stains from vinyl or linoleum, use a dense sponge or washcloth with plenty of soapy water. Try to massage the stain out rather than scrub it out, as you do not want the rust particles to become ingrained.
When you want to remove iron stains from stainless steel, use the finest grain of sandpaper that you can find. Gently buff the area until the stain is just lifted. Apply sliced onion or mineral oil to any part of the surface that made contact with the sandpaper. Clean the area with soap and hot water; dry; and then polish with your favorite stainless-steel polish.
For stone, concrete, brick and granite, use soapy water and a brush or firm sponge. Again, the goal is not to scrub the iron rust particles into the surface, but to lift them away. Use firm, circular motions, but do not scrub. Rinse liberally.
Some of the rust removers that are marketed for metallic surfaces are actually quite fabric-safe. If using commercial rust stain removers, read the instructions and warnings thoroughly. Before using any method to remove iron rust stains from fabrics, test it on an inconspicuous area and allow it to dry. You don't want to do further damage.