If your cast iron pan starts to show signs of rust, it might be the result of not immediately drying the pan after cleaning it. Another reason could be storing the pan improperly. Besides that, your cast iron cookware is handwash only. Do not put in the dishwasher.
Solution:
Use a soft brush or fine sand paper to remove any rust spots, then rub the area with cooking oil. Store cast iron cookware in a dry cupboard. If items are to be stored for a long period of time, place a paper towel between lid and casserole to absorb moisture.