So I've started in on paint prep for my '72 GT. Beginning at the front I'm sanding and filling dents in the belly pan. Some rust, pretty beat up. PO must have hit a lot of car stops. Hard work sanding on your back. As I move to the nose (removed bumper) I'm finding a very thin layer of what appears to be corrosion below the original paint and primer. This is in areas that are dent and scratch free. Could the steel have had this thin layer of rust when painted at the factory 30+ years ago? Or is this corrosion migrating under the original primer? At this point I'm not prepared to completely restore this car and I'm wondering if put on a temp coat of paint will it make things worse.