Over the years I have seen a couple of shotguns and rifles whose metal finishes were destroyed by surface rust.  The Arms, cold from Hunting, were placed in padded gun-cases and not viewed for days or weeks.  They were brought inside and ignored until the owner got around to cleaning them.  Condensation forming on the metal surfaces did incredible mischief.  I had a harmless lesson yesterday.  I live in a century old Farm House on a field stone foundation.  My basement is not too humid because I run a dehumidifier and a fan constantly.  I had a couple of open padded gun-cases laying on a shelf in my basement.  The cases were dry as I slipped Krag rifles in them for the trip to the Range.  At the Range, the closed cases baked in the Sun for 15 minutes as I hung targets and got organized.  When I opened both cases, the Krags inside were wet, like they had been wiped with a damp washcloth!  My theory is that the case padding absorbed and held moisture out of my basement air.  The dark cases, laying in the hot Sun, began releasing this moisture, which condensed on the guns (wood and metal).  Fortunately, this was discovered immediately, the Krags were wiped down and no harm was done.  The moral of the story:  Keep your gun cases and powder dry!  Don't store guns in padded cases.