Flexible Urethane is brilliant
But I also recommend it as an easy option for gluing to. Shoe Goo or E6000 will be flexible and slump in to the canvas. You can buy SHoe Goo in clear and in black so potentially you could just use Shoe Goo if you want a slick smooth coat.
Knowing the project does help narrow down potential options
I am an extreme shoe make overerso will use solvent to unstick soles and make new vamps and tongues and everything
Using shoe grade materials you can make very sturdy and practical shoes:
http://www.arrayedindreams.com/costume-portfolio/sci-fi/mikka-commander-shepard-mass-effect/
These shoes are silver with interference violet and built out of leather plated over a canvas shoe base (they look ridiculously tall but I can run in them and climbed that tank in them
http://www.arrayedindreams.com/costume-portfolio/sci-fi/samara-mass-effect-2/
Those shoes are built in to the bodysuit- easier as that was very stretchy rubberised scuba fabric
http://pinkdiamond.livejournal.com/1250550.html
http://www.arrayedindreams.com/costume-portfolio/sci-fi/star-wars/darth-talon/
The first version has boots where I reshaped the toe, and then covered in another faux leather look fabric.
http://www.arrayedindreams.com/costume-portfolio/sci-fi/star-wars/slave-leia-metal-bikini/
These were slouch boots I cut down to sideless slippers then rebuilt and wedged the layered in to the soles
http://pinkdiamond.livejournal.com/1129139.html
Re-covering is easy if you use a good solvent
The Wapro "stripper" is insanely good. I've also accidentally delaminated leather that was glued very well when i used fuelite on it. I really need to do a proper "hard mode" shoe tutorial, but it's pretty easy so long as you remember to make sure you don't hack in to support, and that a lot of fabrics mimic materials already used in shoes. Also a lot of pleather takes Rub n Buff very well for a stretchier silver effect