In the 70's when I was in the reserves our weapons would be turned into the armory after a weekend drill. They would be cleaned, inspected, and a light coat of oil would be applied before they were turned in. The next month upon checkout they would be coated with surface rust and needless to say our C.O. was anything but happy. After a couple of months of this is was discovered that the armorer was spraying the weapons with WD-40 after they were turned in and it was promoting the surface rust. The use of WD-40 was stopped immediately and since that time the only thing I've used it for is help in breaking loose rusted bolts and nuts.