For in the field repairs, ambroid cement and some small pieces of canvas. Cut a patch large enough to cover the rip/tear, glue it in behind the canvas skin, not to the planking, just to the original canvas skin, might need to open up the original tear a bit to get the patch piece behind it, then add more glue and try to get the tear flat on the new piece behind. Of course, the canvas needs to be dry and that can take awhile. Duct tape is always in the repair kit.
Filler is put on the canvas to fill the weave after stretching new canvas. I make my own, enamel paint, boiled linseed oil, silica flour, japan drier. Cures in 4-6 weeks. I use the Ambroid cement when wrapping the stems with new canvas to help give a watertight seal and assist the canvas to lay flat on top of itself. I have 3 more to canvas soon, starting to get the hang of it.