I've used this filler - I got the protocol from Chris Merigold. The product is Cecofill, which is an aircraft dacron filler. You order it from Tom MacKenzie. You need two quarts to fill a canoe.
Procedure is this. Wet the canvas to defeat the sizing (lubricant used to assist in weaving the canvas), and apply the first quart of Cecofill. Dry overnight, sand lightly and apply the second quart.
At this point there will still be some weave showing in the canvas. Mix a 50-50 batch of exterior latex primer and lightweight spackle compound and apply. When dry, sand. Repeat until you are satisfied with the smoothness and then paint as usual.
There is no mildicide in the filler, so consider treated canvas or treating it with something first.