Why not just purchase a product that's meant for use on fabric as a water proffer, or repellant. There's a lot of them available, they are in a spray can, can be used on wool or cotton, don't damage the feel of the material and last a long time. Mohawk industries makes one, (you can find them on line) and it only takes about 1 oz. of liquid per yard of material.

I use to use this product commercially to waterproof, and stain proof new couches for my customers. It (and many other products like it) will almost turn a tissue into a bucket with out changing the feel or look of the material.

Several products of this nature can be found in the sporting goods area of a store, or in a fabric shop.