Originally Posted by francophile

Most important concern is reducing humidity. Anything consistently over 55% humidity in a closed non-circulating area and your leather seats are going to turn into a mold-fest. Anything over 70% humidity and you'll mold out even with circulation.

I found this out the hard way, rendered two of my favorite saddles unrestorable. Once mold sets in really well on a leather seat, you're screwed. I tried everything, and when I got super desperate I took one and soaked it a mold and mildew blocker, the other in highly diluted bleach water for days. No dice. Even after finishing, drying them safely and fully in a low-humidity area, mold still had roots deep in the leather and it was molding again a few days later.

Mold sucks.