I am a sucker for cast iron. Skillets, pots, Dutch ovens, or griddles, you name it, chances are good I have a few in my collection. I spend more time than I probably should scouring flea markets, thrift stores and yard sales for old iron.
When looking for deals, I keep a particular eye open for pieces that have been neglected. They might have a bit (or a lot) of rust, caked on crud from years of use, or burnt on food from cooks gone bad. These pans can often be had for a song, sometimes pennies on the dollar for what they are worth.
There are several ways to restore the old pots to usable condition. Electrolosys tanks, self-cleaning cycles on the oven, camp fires, and spray on oven cleaner are all often employed to clean old cast iron. I have tried them all, and all of these methods work, they just take a long time and make a mess.
Far and away the quickest and easiest way I have found to make an old pan look new takes about three hours from start to finish. It requires a drill, cordless or electric, and a stiff round wire brush that can be chucked into said drill.
To start the process, find a nice handy spot outside (trust me, you don't want to do this in the house), put on a pair of safety goggles to keep flying bits out of your eyes, and go to town. Spin the wire brush at full speed, wiping away any rust and debris from the surface of the pan.
Once you have everything cleaned off, take the pan in and wash it well with hot water and steel wool. Preheat your oven to 250 degrees. Dry the pan well and heat it in the oven for a few minutes until the metal is warm. Rub a very thin layer of a vegetable based shortening like Crisco brand over all surfaces of the pan. You don't need much. Anything more than a thin layer will result in a sticky finish instead of the slick surface you want on your pan.
Place a cookie sheet on the lower rack of your oven and place the pan, upside down, directly on the upper rack over the sheet pan. Heat the pans for one hour at 250 degrees. Take the pans out and wipe them again with a paper towel with a small amount of Crisco. Return the pans to the oven, still upside down, and increase the heat to 350 degrees. Bake the pans for one more hour.
At this point, turn the oven off but leave the door closed. Let the oven cool down until you can handle the pans comfortably. The finish should be slick and slightly oily. As you use your restored pan, the seasoning layer will continue to darken and get slicker with time.