Update: disagreed with everyone who says rusty cast iron is a lost cause, and a $4 soak proved them wrong
Against all the "just strip it" advice in this group, I filled the sink with warm water and a splash of white vinegar for 20 minutes on a $6 flea market skillet from a yard sale in Boise, scrubbed the rust off with a wadded ball of foil, and it came out gray and even with the "before" marks still showing where it was, so now I'm wondering if anyone else skips the lye bath and goes straight to vinegar for light rust.