Board Thread:General Discussion/@comment-17711916-20140309175011/@comment-11051098-20140320170629

Take a look at chapter 559 page 14 (on mangapanda). Cang states that "The Iron" makes it easy for him to stop a blade like Haschwalth's. While it's not a "proof" in pure words, it suggests that his ability which he used there, covers his skin with a layer of iron.

I would say it's proof enough, being in the manga Kubo himself draws.