Will constant use of a kydex holster eventually wear out the tennifer coating on the slide? Would a leather holster be less damaging to the finish? Also, is it necessary to apply a thin layer of oil on the slide to prevent it from rusting? I'm concerned that due to frequent handling, the sweat from my hands may corrode the slide.
My P99 has around 6k+ rounds thru it, along with hundreds of draws from kydex holsters.
The black finish has polished up a bit on the edges of the slide, but the finish has held up extremely well. The polish just gives it character.
The black finish on the slide is not the tenifer, tenifer is not a "finish" per se, but more of a metal treatment, that permeates the metal. In order to damage the Tenifer you would actually have to remove several thousandths of an inch of the metal itself, not likely unless you drag your P99 behind a car or something.
Unless your sweat is worse than several hundred hours submerged in seawater, you will never see any rust on the P99's slide.
As mentioned above, wipe it down occasionally with a silicon cloth, and don't worry about it.