For those of us who love and use & collect cast iron cookware,there is nothing worse than going to a estate sale or a old barn or atic and discovering a antique piece of cast iron in utter disrepair.Rust,dirt,cobweb's, so bad that you say to yourself that this once fine piece of cast iron can never be usable again.Well in some extreme cases you may be right if there is warpage or cracks visible or a piece missing.On the other hand if none of that is present then lets talk about trying to save this valuable piece.First lets get it home and clean it up to see what we have to work with.Warm soapy water will wash the dirt off. If extremely dirty or full of dirt use the outside hose.Otherwise the kitchen sink will do fine with a good plastic scrubber.At this time dont worry about the previous seasoning if any we will redo this at a later time.Scrub it down really good removing everything that is not original metal.Dont let the rust bother you either at a later time we will fix that too.When you have a very nice clean and probably very rusty piece of cast iron we will move on to the next step.If it is a very light coat of rust covering the piece a very fine steele wool will take most of the rust off and will not harm the piece.Remember we are going to reseason it anyway.Now if you have a piece that has a very large amount or rust.Use the methods above to remove as much as possible.When you have scraped and scrubbed and washed everything of that will come off. Now we will move to the next step with the stubborn stuff. The key to this step is a self cleaning oven.Place your piece of cast iron (NOT ANY PIECES WITH WOODEN HANDLES OR ANY WOODEN OR BAKELITE PARTS) in your oven if it is a pot or dutch oven place it upside down and seprate the lid and bottom on different racks if possible.Now close your oven and start the self cleaning cycle.(you may want to open a window or turn your vent fan on this process can cause some slight smoke and fumes in your kitchen as the old finish and rust is burnt away).This process usually takes several hours for the self cleaning cycle to finish.Once the self cleaning cycle has run its course you should now have a nice clean oven and a clean new piece of cast iron.Do not immediately remove you cast iron from the oven. Cast iron does not like extreme temperature changes and can warp or crack when exposed to them.Let it cool naturally as the oven cools down.Once cooled to where it can be removed with the bare hand.You will find it bare metal with a dust looking outer layer covering the piece.Now the tricky part this dust will be in every nook and cranny of this piece and will need to be removed before you can season this piece.A air compressor works great or a quick rinse with hot water before reseasoning.If you use the rinse method you will need to dry it very quickly.Bare cast iron will rust before your eyes.As soon as you have the dust layer removed and the piece wiped down coat this piece with oil (Crisco,or peanut works best for me).Coat the piece througly with oil covering it and make sure you get it into every crack and crevice of the piece.(Use a brush if necessary)You want it covered but not dripping this will prevent rust and also prepare it for the next step which is seasoning.Now you have a clean piece of cast iron with a fresh coat of oil lets heat the oven back up.I personally heat it to about 450 degrees.Now as your oven heats up place the piece of cast iron back into the oven again if it is a pot or dutch oven place it upside down on the oven rack with the lid alongside or on a different rack.You may want to place a piece of tin foil in the bottom of you oven to catch any excess oil that may drip out dureing seasoning.Again open a window turn on your vent fan or ceiling fan this will usually smoke and smell for a short time.Leave the piece in the oven once it heats to temp set your timer for about 1 hour  or a little longer wont hurt it.Now repeat what you learned before once the hour is up turn your oven off and let it set and cool before you remove your cast iron.Once removed it should have a black glossy finish.If not repeat the process starting with another coat of oil and back in at 450 for another hour.Now it should be ready for cooking in or displaying.Now hopefully your non useable piece of cast iron has been reborn into something usable and beautiful again.I hope this helps someone. I do not claim to be a expert in cast iron but have used this process successfully many time.Please be careful and remember to use adult supervision when needed.Always use caution around a hot oven or pots.There are also several very good dutch oven groups online that are free to join and have a wealth of information to share please look into some of these groups if this interest you.Best wishes and better cooking with cast iron.