Found this on-line which suggests cream of tartar and hydrogen peroxide.
A homemade paste of hydrogen peroxide and cream of tartar can be used to remove rust stains from fabrics or hard surfaces. Simply pour a couple tablespoons of hydrogen peroxide into a bowl and add cream of tartar until it forms a thick paste. Spread the paste over the rust stain and let it sit for 30 minutes. The acid in the cream of tartar and the oxygen bleach in the peroxide should ensure that when you rinse the paste away with water, it takes the rust with it.