The mim thumbpiece or cylinder release as I call it can easily rust with pocket carry. I had this happen to a 442, and I'm trying to prevent it with my 642. I am going to coat my 642 with MPRO7 CLP as S&W customer service recommended, but I don't have full faith in any CLP. What I have come across is the Desantis Superfly holster which I believe will solve the problem. The holster comes with a velcro attached flap which is supposed to minimize printing, but I'm using the flap as a sweat shield. You attach the flap as high on the holster as you want so that only the rubber grips get sweat on them. My wife can't tell I'm carrying with the flap near my legside of the pocket, and I don't see any more printing than what I had with my old Desantis Nemesis holster which does not have the extra flap. As a side note, I have a set of Hogue Bantams on my 642 which doesn't have a grip screw. So, nothing on my 642 gets exposed to sweat/rust anymore. I just wanted to share my latest holster find. By the way, I am not affiliated with Hogue or Desantis. I just wanted to share how I eliminated a rust problem with my 642.
