Canvas "duck" is waxed or "oiled" canvas. Sno-Seal should restore that wax. It's marketed for leather but says right on the can that it can be used on oiled canvas.
Just did a quick search and came up with two products specifically for the job: Filson's Wax and Australian Outback Dry Feel Reproofing Cream. They both sound pretty much like Sno-Seal. They're the opposite, marketed for canvas but said to work on leather. I reckon they're interchangeable products.
http://store.thewharfonline.com/work...wax-p-882.html
http://www.australianoutback.com/?cP...d=9900&tpid=79
Just like Sno-Seal, warm up the thing to be re-waxed and warm up the wax and go to town.
Chris