My spouse is sort of a "foodie," and very particular about her cookware, including the rusty cast iron (new or old). She basically scrubs the crap out of it with plain steel wool to get the rust (mostly) off, then she rinses off the leavin's with water (no soap), wipes it down with a paper towel and immediately applies cooking oil to it, and then puts it in the oven at around 350 F for an hour or two. I think she got that technique right from Lodge, but I may be wrong about that. She is one of those people who considers removing any "seasoning" to be a high crime.
I once suggested using Ballistol on one of her skillets, and if looks could kill...
(It is supposedly a disinfectant, burn ointment, emulsifier, insect repellent and who knows what else, so I thought I was just being helpful. Wrong. Again.)