Do not use Thompsons Waterseal. It is no longer rated for use on canvas. It once was, but is no longer, intended for canvas.
Lowes Olympic is still usable on canvas, but you will have to use a seam sealer on any seams in the shelter. Someone talked about a solution from Ace Hardware the other day, but I have never used it.
Tipis had a list of solutions she uses on the shelters she builds just the other day, on another thread.