I'm going to assume that you have no general humidity problems or it would affect your others pins. You said you had just finished restoring your pin so I'm going to suggest a possibility, well more like a guess. Perhaps your clear on the PF is still giving off gas and that gas being trapped under the glass has to at least have marginal water in it you're getting rust, or it's a pure chemical reaction to something in the gas. Probably far-fetched but I thought I'd offer it up. Maybe one of the other restore guys have seen this or can chime in.