On my recent trip I needed to spray my tent again with some waterproof spray. I use Tent Mate which I buy from my local camping shop. The 500 ml bottle ( for $20 ) does my tent twice, and a new bottle does the camper trailer once.
The idea is to give your canvas a lite spray, just enough to bead up and not run. This stuff is great and you can use it inside and out. There is a host of anti mould - water proof agents out there, but I find this works for me.