Unrust Stain Preventer Unrust Stain Preventer prevents irrigation rust stains. 100% concentrated chemical in the dry form. 1 eight ounce cup of dry UNRUST Stain Preventer will make 1 gallon of ridorust liquid. This 1.25 gallon size holds 20 cups of Unrust Stain Preventer. You save money when you compare how much you would spend for 20 gallons of ridorust.