The beautiful thing about cast iron it that it can last virtually forever if it is well cared for. Even if it has not been properly treated it can most often be easily restored to near mint condition. Here are a few tips for restoring your cast iron cookware.
The first thing you want to do is to examine the piece and check for cracks or deep pitting. If you do find cracks the skillet or pan is most likely not worth the attempt it would take to repair it. It is rare for cast iron to crack but occasionally it does happen. When cracking or very deep pitting occurs it can be costly to repair as it requires an expensive process known as brazing. Brazing leaves the pan useless where cooking is concerned as food can be contaminated when prepared in brazed cookware. So the best thing to do discard it if cracks are present.
Now that your cast iron piece has passed inspection it is time to begin the cleaning process. You will need to remove loose particles such as rust and food build up with a wire brush or steel wool pad. Scrub away as much of the loose debris as possible.
Next you will need to make vinegar and water solution. It should be one part vinegar to 10 parts water. Soak your cast iron cookware over night to loosen and dissolve the leftover rust. You don't want to use any type of chemicals on the cookware as chemicals tend to soak into the iron and this may lead to food contamination when you cook. If the piece is extremely rusted or dirty you may have to repeat the above cleaning steps a couple of times to achieve the desired results. It is better to put forth a bit of extra work than to take the easy way out using chemicals that may later harm your family.
The last step is seasoning. Seasoning protects your cast iron and makes it easier to clean after use. Thoroughly coat it with lard or solid Crisco. Avoid liquid oil as it tends to leave a sticky residue. Place your cookware in an oven that has been heated to 300 degrees. Bake for 15 minutes. Remove from the oven and pour out any melted grease. Return the cookware to the oven and continue baking for 2 hours. This process leaves your piece with a protective finish. May be re seasoned when needed.