I have been making holsters, that is, ostly "one-offs" designed for specific guns for 25 years. I have never had a finish effected by the form fitting, and almost all of those guns were simple blue guns. And I don't even make much effort to oil them before fitting/boning them.

I use a gun in a holster every day, or nearly every day, in perfect weather and in total messess; dust {now}, rain, snow, sleet, etc.

Will the tanning acids effect a handgun left in a holster? Yes, over time they will, especially in a humid environment or if the gun or holster is wet when they are mated and then left together. But in practical terms, I do something occaisionally that seems to mitigate the problem...I clean the gun. And I don't store guns in holsters...or maybe I do. My .44 has been stuffed under the seat or in the bed of the Polaris Ranger or on my hip for a month now...

Here's my .44 with me yesterday, building a bear blind {hence the bolo}.

I do the following when making a holster for a friend, etc...

Upon finishing {after fitting, with the holster bone dry}. I SOAK it in in silicone spray. SOAK it. On a big rig, I'll use a whole can of silicone found in automotive stores. I let that dry maybe a week. It makes the leather almost feel like rubber. I leave it at this stage and just use it or...

Then, I heat the holster to 120 degrees F and ditto for the SnowSeal, and apply that liberally, allowing it to soak and dry-in.

Then...I use it. This last bit is the only thing that some folks never get to. I use stuff, wear it out, remake or make new, and don't mind a scuff here or there. I won't live forever on this planet, and neither will my holsters or guns. Wear indicates use. And that indicates experience.

Enjoy!