Unrust Stain Preventer Unrust Stain Preventer will stop your rust stains caused by iron in your irrigation water. This size of Unrust Stain Preventer contains 5 cups (40 ounces) of 100% concentrated rust prevention chemical. All you need to do is add the contents to your chemical system you are using and add water. This size is the prefect size to try Unrust Stain Preventer to get rid of rust and Save you money on more expensive chemicals. Unrust Stain Preventer will stop irrigation rust.