You want to use something that will still allow the canvas to breathe. TWS is not good for canvas tents.
You could just set it up on a bright sunny day and wash it down then let the sun dry it and shrink it back up tighter. Then you can just use a tarp like stated above for a rain fly.
I would get the stuff Davis sells and use a tarp for a rain fly. You'll be dry in a driving rain then.
