[Philip]

The Edster is right about that scabbard. The tannic acid in compounds typically used to process raw hides into finished leather leaves traces in the material that cause unprotected iron or steel to stain or rust when left in contact over time. This is especially true with leather made in traditional societies in which the processes were not uniform or brought to a high standard. But you see this corrosion problem even on knives and swords from the industrial-era West as well. The general tendency is for blades to corrode worse when kept in all-leather sheaths, as opposed to metal or wood-lined scabbards.

Oiling the blade carries other risks, beginning with the tendency of leather to absorb it, leading to a darkening and softening of the material. Excessive use of oil causes the leather to eventuually rot and disintegrate.

The best remedy is to store your kaskara out of the scabbard. You may want to clean out any active rust from pitted areas (without refinishing the whole blade) and preserve it with a good wax. A common problem when storing and handling all-leather scabbards without blades in them is that they can become kinked since these sheaths have little sideways resilience now that they are a century or more old. Simple remedy is to shape a wooden lath to the same dimensions and shape of your blade, leaving it a few inches longer to serve as a handle for easy removal, and keep that in the scabbard so that it stays rigid and straight. Best to use a non-acidic deciduous wood like poplar or alder, which are relatively soft and easy to work, and won't react with the leather. (pine, cedar, and other woods from coniferous trees have a high acid content in their sap which makes them less desirable for the purpose).