For all the work you'll put in including the coatings, I think you could buy a yard or two of Cordura or any waterproof fabric for very little at a fabric store - save yourself a lot of hassel - easier to work with - and have a better end result. You could certainly do it if you want.

I've done things like that in the past and about 3/4 the way through it I'm wondering... why am I doing this?