I have done three canoes in the past year and a bit using lagging compounds like the stuff that Rod Tait recommends. Indeed, the first canoe I canvassed with this material was with the Robson's which I bought from him. For the other two, I used Bakor 120-09 lagging compound. The two materials are nearly indistinguishable for me, and I was able to get a 20L bucket of the Bakor locally (eastern edge of Canada) for only twice as much as the 1 US gal can of the Robson's once shipping had been figured in.
The 20 L bucket covered a 14' Chestnut Fox and a 16' Bastien, provided a US quart can each for repairs and left nearly 10 L, which I gave to a friend who has another Bastien.
I know nothing about the longevity of the products, but I have already repaired a spot where the Bastien ground over a sharp rock. The repair was dead simple - paint it on, wait a bit, paint on some more, repeat if necessary, paint.
Initial application is also dead simple. I painted it on the canvas and used a grout sponge to try to force it into the weave, let it dry, then painted on more coats until I was happy with the result.
I did find that the first time I used the lagging compound, 1 US gallon did not provide enough to smoothly cover the weave of the cloth on a 16' Chestnut Prospector. Another couple of cups would have been perfect. The Prospector is quite deep, though and I'm sure there is enough in the can for most canoes. More skilled application might make a difference, too.
There is nothing either toxic or even unpleasant about these lagging compounds and you can have the canoe in the water a lot quicker than with traditional filler materials. A month to cure! Try four hours.
Ron