Like I said the canvas needs to breathe.
Use the right treatment if you must do it.
Wetting and letting the shot sun dry it out will shrink the canvas and help it become more water tight.
A cheap poly tarp works just fine as a rain fly too, maybe a cheap canvas drop cloth.
This, my tent is only 25 or so years old! Every couple years i put it up in the back yard! Saturate the canvas with water and let the sun shrink it tight!
