How to Choose a New or Used Pan
When it comes to cast iron, you can choose new or used, enameled or non-enameled pans. Used pans are likely to come from a yard sale, flea market, online auction, or as a gift. Enameled pans are cast-iron pans that have an enameled coating, usually on the inside and out. This coating is often white on the inside and comes in a variety of colors on the outside, depending on the manufacturer.
Brand New and Dull
When buying new cookware, look for pans that don't have any large pits, bumps, or cracks on the cooking surface. These imperfections can make it harder to cook with your pan. Ideally the pan has a similar texture to that of a piece of high-grit sandpaper. The pan should be dull gray and it shouldn't be smooth. As you cook with it, it will become blacker and smoother. And finally, you want to make sure that the thickness of the bottom and sides of the pan is even. Pans with bottoms and sides that get thinner around the edge are less likely to conduct heat evenly and more likely to break.
Old and Shiny, or Old and Rusty
Cast-iron pans, if cared for properly, get better with age and use because they become more seasoned with each use. Even an old pan that is rusted or covered in bumpy residue can be salvaged, and will likely be better than a new one. You need to sand off rust with steel wool, large grit sandpaper, a rust eraser, a small steel brush, or even just scouring powder. Thick carbonized residue can be removed by placing your skillet in the oven during the self-cleaning cycle. You can also spray the pan with oven cleaner, place it in a plastic-bag, and let it sit for a few days. Wash the pan thoroughly using soap and water and scrub the affected areas with steel wool if necessary to get back to the bare metal.
You can ruin a cast-iron pan beyond repair by changing the temperature of the skillet dramatically and quickly. If you pour ice water into a heated skillet, or if you have a skillet in the freezer and then place it over a high heat, the skillet may crack.
If your pan has had paint or any thick and sticky oil stored in it, you can use the techniques described earlier to clean it. Make sure your pan doesn't have cracks, even if they don't go all the way through. Look for black epoxy used to fill any pits or cracks. It can be hard to see epoxy-filled cracks, but if you see black paint, ask before you buy if you intend to cook with it. Epoxy is poisonous and will release toxins when heated.