Carrying anything in close proximity to the human flesh will mean its going to be in a high humidity environment. Lots of men - millions, in fact - would carry a carbon steel bladed pocketknife in their pants pocket which would never touch their skin, yet they would rust like the dickens if not cared for.
If anything a gun in a IWB gets more air circulation and suffers less. What did it take for pocketknives to not rust? The only effective answer was to use non "ferrous" steels. Today's knife doesn't rust - it's likely a polymer or synthetic handle with stainless steel blade.
There's your prescription. Close carry means no carbon steel, and that is why so many less expensive guns aren't going to make the grade in the long run. Just the same as those old pocket revolvers we see at gun shows with obvious pitting and finishes long gone. Take a look at the service revolvers of those days, too - same problem when an officer carries a carbon steel gun in inclement weather in a leather holster. The stainless service autos that replaced them don't show nearly as much corrosion.
Carrying concealed is a lot like placing a gun in a "marine" environment where high humidity, oils containing salts, and continuous exposure for over 12 hours a day takes place. Very few of us would deliberately store a gun with ferrous parts in the hold of their boat sitting at the dock, yet we do expect the same gun to resist an even more active environment stored in a holster near our body and constantly bathed in an atmosphere of corrosion.
I won't even discuss what you are doing to a gun after a lunch of bean burritos. Let's just say methane from cows is getting regulated in California. : ) The deeper the carry, the harsher the situation. Polymers and stainless do a good job but when the gun maker value engineers the material selection on the small parts you still have a problem.
