How do I clean rust and cooked on food off cast iron grill grates?

My parents gave us a grill last year, and they stored it outside and never really cleaned the grates very well, and my husband and I are wondering if there is a way that we can clean the rust and really cooked on food off our cast iron grill grates?
Is there a particular product or method for doing this. We would like to save as much money as possible.
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