Cast iron grill grates age over time and need proper care to stay in good condition. Burnt food particles, grease and other debris accumulate on the grates, preventing them from retaining their natural nonstick properties. If you leave the grates outdoors during the winter or don't use them for long periods of time, this may cause them to rust or grow mildew. The grates can usually be cleaned and restored. However, you may need to replace the grates if there is a large accumulation of rust that you can't remove or if the rust has eaten through large portions of the grate.
Scrub the cast iron grill grates with a dry wire brush to remove rust, burnt food particles, mildew and other debris. Scrub off as much as possible, and use a metal scraper to chip off large chunks if necessary.
Coat a paper towel in vegetable oil. Rub the grill grates with the oil, and continue wiping and rubbing the grates until they come clean.
Soak another paper towel with white distilled vinegar and use it to remove any remaining rust spots. The vinegar is not necessary unless the rust does not come off with the oil.
Apply another layer of vegetable oil to the grates. Preheat your oven to 300 degrees Fahrenheit, or preheat your grill on low.
Place the oil-coated grill grates in the preheated oven or covered grill for 30 minutes. Check the grates and apply more vegetable oil if they dry out. Return to the oven and check them every 30 minutes for one to two hours.
Wipe the grates off with a clean, dry paper towel, then apply a final thin coat of vegetable oil. Turn the heat off. Leave the grates in the oven or grill until they cool completely.
Store the grill grates indoors in a dry location when they are not in use.