Cast Iron Scavenger
My husband dragged home a sad-looking but perfectly-sized cast iron skillet with lid that he found at a flea market.
I gave it a good scrub with steel wool, but I can't seem to get all of the rust out of the crevices on the lid (around the lip, primarily, which should not come in contact with food, theoretically). Also, the top of the lid has a dimpled surface that has some black finish in the indentations.
Should I go back at it and scrub it perfectly clean (although I'm not sure it's possible, at least by the means I have at home)? Or is it okay to season it as is?
Put it in a self-cleaning oven. It will take all the crud off and sterilize it as well. You have no idea where it has been.
We did this with all the family cast iron that had been submerged in Katrina's floodwaters for weeks. Didn't even want to think about what was in that water. Of course it removed decades of perfect seasoning as well but the pans are all back to normal now.
After the self-cleaning cycle is complete, wash the pan and lid well with soap and water and season just as you would a new one. It will be fine in no time at all.
Adding to what ccbweb said, I've had a lot of luck heating my cast iron on about medium for, say, 5-8 minutes, adding a couple of tablespoons of vegetable oil and a generous pour of kosher salt. Scrub hard with several papertowels folded over a few times (helps to have an oven mitt for your non-scrubbing hand to maneuver the pan). The salt is abrassive enough to cut through the rust without damaging that mercurial pig iron. Works great on hard to clean cast-iron grill pans as well.
You might also try adding some kosher salt and a small amount of water and giving it another go with the steel wool. The salt will get into some of the crevices. Though, the oven cleaner thing will get the job done and since you're going to have to wash the thing and re-season it anyhow, maybe the easy route is best.
Spray it liberally with Easy Off oven cleaner, place in a plastic bag, seal the bag, and leave it for about a week. Then hose it off in conjunction with a wire brush and season immediately.
Hosing best done outside. If you have a small pressure washer that will work even better. The usual precautions about using oven cleaner apply.
I received one in the same condition. The dimpled surface was actually years and years of baked and caked on residue that had turned black. I actually used my high speed Dremel tool with a buffing attachment that actually got off all of it. The pan almost looked new. Then I seasoned it and use it all of the time. I would suspect that you could find some sort of wire brush that would accomplish the same thing.