I have used
Pellon iron on vinyl fuse before and it does make fabric water and stain resistant. I have used it in zipper pouches, lunch bags, sandwich bags, etc. You iron it onto your fabric before construction and can be used inside or out. Just follow the directions on the package. Very easy to do. Sews beautifully too.
It is available at Joanns, Walmart, Amazon, and others.
http://www.pellonprojects.com/products/100r-vinyl-fuse/