I use SnowSeal, a silicon impregnated beeswax that works great for waterproofing leather. I treat all my leather knife sheaths with it, inside and out (I even take them apart), melt it in with a hot air gun or hair dryer. It prevents the leather from absorbing moisture, which is the #1 culprit in causing rust on blades (at least, those stored in the sheath), and doesn't soften the leather. Looks good, too.