I thought I would share my thoughts in regards to cast iron. Hoping it might help at least one person who may be having problems. As an example, I purchased these 2 skillets for 10 bucks each. They were a mess. I saw no cracks in them so no problem. And they weren't spinners (which means they were warped on the bottom and then when you put it on your glass top stove they spin!!) Here is what I do when I bring in a new/used cast iron skillet into my house:
First off, I get some Easy Off oven cleaner (yellow cap!! The Easy Off with the yellow cap is heavy duty and that is what we need) ) and go outside & spray the heck out of it, inside and outside and completely cover the entire skillet. Use gloves and eyeglasses. Once coated, put the pan in a plastic kitchen garbage bag and tie it up and put in a warm place for 3 days. I usually put it in a cardboard box and close it up. After 3 days, remove pan with gloves and take a stainless steel pad and scour it all over. Rinse. If it still has some issues, I will repeat this process for another 3 days.
When the skillet is free of any debris, it is now time to season it for cooking. This means getting a cooking surface baked onto your skillet that allows food not to stick. Put crisco on a rag and wipe the pan all over, inside and out. Then remove as much as you can using a clean dedicated rag. You just want to leave a thin even coat along the entire skillet. (I keep my rag in a tupperware container in the fridge and use it over and over again.) Put in the oven on 450 degrees upside down (lay foil under where the skillet will be.....I also save the same piece of foil and use it over and over again.) and bake for about 1.5 hours. Let cool overnight in oven. Remove pan in the morning and repeat one more time. This allows you to have a nice cooking surface where your food will not stick. You can even do this 3 times.
After cooking, I just rinse the skillet in water, dump the water out, add a drop of Dawn dish washing liquid and a bunch of kosher salt (used in this case as an abrasive) and scrub it gently with a dobie pad. You don't want to scrub too hard and risk taking some of your seasoning off. If there is any food stuck on let it soak for 10 minutes. Dry completely and coat with whatever oil you use (I use cast iron oil available on Amazon) and put in the oven upside down if it is still hot from dinner. If not, heat on the stove top inside and out and then oil it. And that is another thing; if you used your oven for dinner, leave it on until you empty your food out of the skillet. Then clean it right away, oil it and put it back in the oven with the oven still on. After you eat and get your dishes done, simply turn your oven off and don't open the oven until the morning.
For any rust, mix equal parts water and vinegar, and let the pan soak in the mixture for between one and 3 hours, depending on how bad the rust is. You really don't want to soak it any longer than this because it could ruin the iron. Then gently scrub the rust away from the cast iron pan with your scrubber. Still rusty? Well, rinse it well and then try soaking it for one more hour and scrubbing again.
NOTE: As soon as you take it out of the vinegar bath, it is going to want to rust again. Oil it quick to prevent this.
NOTE: I never advise anyone to put a skillet in a self-cleaning oven. Dumb idea. If the skillet has a lot of gunk on it, it could catch fire in there and then you are screwed with a locked oven door.
THIS IS THE OIL I USE TO LIGHTLY WIPE DOWN MY PANS AFTER COOKING. I JUST USE A DROP. THIS BOTTLE LASTS ME ABOUT 1.5 YEARS AND I DO ALL OF MY COOKING WITH CAST IRON.