All of our cast iron pans are old. I can usually clean them with a green scrubby pad or one of those copper coil scrubby things, and sometimes have to scrape it with a wooden spatula. Usually, a light wash and quick wipe down to dry, and it's good to go. If it needs a lot of scrubbing, then I'll heat it up after drying and re-season with beef tallow or avacodo oil on the stovetop, which only takes a few minutes or so.
If it's really bad, then I'll soak it and even use dish soap if I have to. The only time I get rust is if I let one soak in the sink for a couple of days before I get to it. Then, I get out some steel wool or even light sandpaper to remove it, and have to spend more time to re-season it.
[Edit] For enamel lined cast iron, I start with a green scrubby pad and dish soap, plus a wooden spatula to scrape, if needed. For tough spots, I break out "Bar Keepers Friend", but I try to use that sparingly, especially on the outside, as I don't want to dull the finish, even though Le Creuset says that product is fine.