--Use a stiff nylon brush, hot water and a little salt (a natural abrasive) to scour off the rust. You may have to throw away the salt and add more and repeat the scouring process if the pan is really dirty.
If that doesn't work, use hot water and steel wool. When you're done, coat the inside and outside of the pan with vegetable oil and place it on foil-upside down-in the oven at 350 degrees for an hour to re-season. Allow to cool before using or storing.
--To clean rust from an abused garage sale find of a cast iron skillet soak the item for 24 hours in a bath of 1 part molasses and 3 parts water. Clean with a bit of steel wool. (resoak as needed) Season and voila.
--To prevent future rust: After washing the pan, put it on the stove and let it heat until any water left in the pan has evaporated.
--Never use scouring powder or soap! The soap will suck out the good oils in the pan and the pan could absorb the soapy taste!!