David,
My original, unrestored '69 Z28 (NOR) from 09C also has areas of gray, black, and even some outer body overspray on the low hanging areas. the highest areas under the body generally are gray (ie. didn't get the black as much)... the lowest areas hanging down got touched on the sides by orange *my outer body color*.... I just cleaned it up and left it as is.. although there are spots I'd like to convert the rust, I'm going to leave it as is until it's judged at least once...
Gary