Use a white vinegar solution of 2 parts vinegar to 1 part water to clean the rust off cast iron pans.
When cleaning antique cast iron pans soak the pan overnight in the vinegar solution and the vinegar just dissolves the rust. Very rusty pans many take an extra evening.
Don't leave it in there too long or you will not have a pan. The vinegar actually dissolves the metal. This would work on new cast iron pans also.