It really does no good to waterproof your traditional scabbard.
It is water buffalo hide over a split wooden core.
No matter how much you waterproof the outside leather, the wooden core may still get wet.
The effect of the waterproofing will then be that of sealing in the moisture. It is better for the scabbard to be able to "breathe" and allow this moisture to evaporate. The scabbards are traditionally made in this manner so that the scabbard will expand/contract to the variances in climate.