Otter Wax Water-Repellent Fabric Wax
Wax on, water off. Otter Wax is a canvas wax handmade (and continuously tested) in soggy Portland, OR to repel water from shoes, bags, packs, hats, and jackets. The compound incorporates only natural drip-and-dew-eschewing waxes and oils, containing no silicone, paraffin, petroleum, or other synthetic ingredients.
Sold here in bar form, Otter Wax applies to directly your backpack as you'd apply Irish Spring to your armpits--just rub it right on.