Maybe undergarments make a difference? I clean my SA XDM-45 4.5" only after shooting. No rust ever, and I'm in Houston, where 90% relative humidity is a dry day. Some thoughts. I wear a black "wife beater" T shirt under my open bottom outer shirt. The T helps keep perspiration from the holster and gun (as well as helping to hide the black gun under an outer shirt.) Gun is in a Tucker Secret Strike, an all-Kydex full coverage holster (open at the bottom, of course). I have had leather and half leather holsters in the past, and they seemed to get damp sometimes, which I suspect would keep dampness in contact with the gun.
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I imagine the gun makes a difference. Glocks seem more rust resistant than some others. I have also had no rust on my SA XD's or XDM or my Colt XSE, although I have not carried the Colt much. I seem to recall that Kimbers are notorious for barrel rust--when I've looked at them in gun shop a, they always seem to have a little rust in/on their "stainless steel" barrels.
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I also have an inside job, so perspiration is not that great at work. However, my wife is a nature photography enthusiast, and we spend a lot of time in wildlife refuges, where perspiration is significant.
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In the evenings at home, the gun comes out of the holster, gets a Streamlight attached, and sits on the kitchen counter or a table, until it goes to the nightstand, so any moisture that might be in the holster is separated at night and dried in the dry indoor air.
Sent using Tapatalk...so please excuse the typos!