I have some experience with rusty staples as well as cover oxidation, but not via CGC submission, rather with Mycomicshop(MCS).
I once bought a raw 9.0 from eBay, which when I sold through MCS, was graded 6.5. The book had minor rust on staples!
I bought a raw 7.0 from a boardie here, which when I sold through MCS, was graded 4.5. The book had minor cover oxidation (which was disclosed by the boardie).
What I've understood is that they are notoriously strict when it comes to defects like rusty staples or cover oxidation, and flat out take at least 4 grades off. This would hold for structurally mid to high grade books.
Obviously a book that is structurally 2.5 cannot be graded a 0.5 because it has rusty staples or cover oxidation. That would cause mayhem!