I have used the iron on vinyl. I have used it on a paper ( made a Diamondback bag to take to the games. Made paper copies of shirts, logos, old tickets and even a baseball card of my all time fav player) and on fabric. It comes with a paper you use to keep on top. You only iron on the paper and it peels off when you are finished. All the info was on the paper. Not hard..you just have to remember not EVER to touch the vinyl directly. The paper has to be on top. I would think it would make your flag pretty water proof. You will have to figure out what to do about the binding, so the water does not soak thru from the top.