My personal method of treatment of leather goods, and l have quite a collection of items that date 100 years plus, is firstly clean and rehydrate ever so slightly using one of the branded children’s face and hand wipes, it dries quite quickly leaving the leather clean and ready for the first and maybe only treatment (depending on the condition of the leather) of “Hydrophane Leather Treatment” (blue tin) applied sparingly with a lint free cloth, don’t flood any leather with any treatment, a little at a time until desired effect is achieved, leaving the item at room temperature to be able to absorb the treatment until dry.
If the leather is rotting, (red rot) crumbling, it’s probably due to the way it was tanned, it’s difficult to repair or stabilise this effect and maybe the only way is to replace this section of the leather that’s rotten, unless it’s of Historical importance.
All my leather treated as described doesn’t deteriorate or show any signs of mould as maybe found using a preparation like neatsfoot oil.