Some of the best cast iron that I have has come from Goodwill - people buy it, use it once without curing
it, wash it (probably in the dishwasher) and end up with a nasty looking mess.
1) Before using cast iron:
rinse it off, and dry it on a burner. Coat it thoroughly (inside and out) with fat - solid fat is generally better than
cooking oil, but I don't use shortening, and the animal fat goes into soap: I've never had trouble with liquid oil;
Put the pans in an oven preheated to 200 degrees, bake for an hour, and then allow them to completely cool while still in
the oven. You may need to treat them more than once, and it takes quite a bit of use and oiling before they're well
treated, but once they are it's like using a non-stick pan;
3) I wash my cast iron when I need to - just dry
it on the stovetop and oil it well after you're done.