No matter the source of the rust spots on your cotton towels, these stains that seem to stay put wash after wash need not remain permanent. Natural spot removers such as white vinegar and lemon juice erase those stubborn stains without chemical treatments. If the towels are deeply colored, test your homemade rust remover in an inconspicuous area before applying it all over to ensure it does not discolor the cotton.
A Dash of Vinegar
Set the affected towel atop an old folded towel, or over a washtub or sink. Pour white vinegar directly over the rust stain to saturate the fabric. Sprinkle salt atop the stained area, rubbing it in a bit; then set the towel outdoors on a clothesline or over the back of a chair during dry sunny weather. Place the towel near a sunny window, draped over the back of a chair, if you're unable to take it outdoors. Wash the towel using your usual laundry detergent afterwards.
Lemon juice dissolves rust spots; in fact, it acts as a bleach-free stain lightener on the towels. Pour bottled lemon juice over the rust, or rub half of a cut lemon over the stains. Blot excess lemon juice from the surrounding area with a fresh white cloth. Allow the lemon-soaked towel to air out in a sunny location -- ideally outdoors -- for an hour or so; then wash the towel as usual. Lemon juice may lighten some dyes, so test an inconspicuous corner of the towel first to ensure the towel's color is not affected.
Bypass the Bleach
While it may be tempting to treat a rust spot on a white towel with bleach, resist the urge. Bleach paired with rust creates a chemical reaction that sets the rust stain, making it even more difficult, if not impossible, to remove.
If one or several towels contain quite an abundance of rust spots, soak them in a vinegar solution before washing them. Mix one part white vinegar with two parts water in a large bucket; it may take more than a gallon of each liquid to soak several towels. Mix in a cup of salt for a gallon or more of combined liquid; then soak the affected towels in the solution outdoors or in a well-ventilated area overnight. Vinegar also removes odors and residues from fabric softeners and dryer sheets as a bonus laundry booster.