IMO and experience it's a mistake to make leather waterproof. Leather by it nature is water resistant. Every time you ride the bike the leather saddle gets wet from perspiration. It needs to dry. Waterproofing leather will clog the pores of the leather and will not let it dry. Mold will start. Waterproofed leather will also get wet anyway, from humidity, fog etc., then it won't be able to dry out.