I would not call it waterproofing,,,, but I use Corbin Saddle Lotion on all my oem and aftermarket bike seats. For preventing dry cracking and UV damage,,, it is pretty effective. Wifey is going to stitch me up a seat cover from a reflective ripstop nylon this coming season. Not sure how rainproof that will be,,, but the full cover usually comes out over night anyway. Hope you find what you need. oh,,,,
www.corbin.com for the lotion. Cat'