Cool Tools! Nikwax Aqueous Wax - Waterproof leather/synthetic sealer
http://www.kk.org/cooltools/archives/003511.php
Unlike most waterproofers, Nikwax Aqueous Wax can be applied when the leather is wet. On one particularly nasty dog sledding trip, I tried reapplying Mink Oil and Sno-Seal, but the leather was already soaked and neither would penetrate. The rest of trip was miserable. For my toes at least. Not an issue with this stuff. Since Nikwax Aqueous Wax is water soluble, it naturally concentrates in areas which are more porous and prone to leak. It's available in brown or black, and more or less restores your boots to a factory look, not greasy at all.
