If you have a rusty cast iron skillet or dutch oven the easiest way to remove the rust is by running it through the self-cleaning cycle found in most modern ovens; by the end of the cleaning cycle any rust is turned to ash. This effectively "resets" your cookware and you will have to season it before you cook with it.
Permaculture site Rich Soil maintains that this method is easier and way more effective than using sandpaper, wire brushes or chemical cleaners. The cookware may not look great when you first take it out of the oven, but after washing off all the ash it should look almost new. As the self-cleaning cycle might discolor removable oven racks, you may want to consider placing the cookware on a brick or ceramic coffee mug to keep it off your oven's coils as shown in the photo above.
I tried this method out a year ago with an old Griswold cast iron skillet that belonged to my grandmother but had languished in a garage for a few years after she died. The skillet was covered with surface rust, but the self-cleaning cycle took care of that and after seasoning the cast iron skillet it has since become my favourite pan.
using a cast iron skillet ain't so hard [Rich Soil - Permaculture Articles by Paul Wheaton]