I don't see this very often on the net but it's pretty comon here in Sweden so I thought I should mention it.
I started canoeing during a 2 years course at Sjöviks Folkhögskola in Sweden. It was a bushcraft course with very big emphasis on canoeing. That said, we used the canoes A LOT, so they needed to be strong but still light enough for the average person to portage.
The solution is to treat the canvas with about 3-4 coats of tar, applied in spring when the sun is strong but before it's too hot so the tar starts dripping. It needs to be rubbed in deeply in the canvas. We usually did it with big brushes. But be careful not to put too much, it will take forever to dry. It's okay if the first coats look uneven. Depending on the weather you might have to wait between 5 days and a few weeks between coats. I would say usually around 1 week. Let it dry by a south wall outdoors for quickest drying.
When the tar has dried you're ready to apply shellac. The best is to use "orange shellac"-flakes which are not dewaxed. Then mash them up into small pieces and mix them with denatured alcohol (one third shellac flakes to two thirds of alcohol). It takes a while to dissolve and you need to shake it from time to time. When it's all liquid you're ready to go.
Apply it under the water line. I recommend taping the water line for a straight line. You will see that it dries very quickly. And for each layer it will be quicker and quicker. But don't stress it. For the first 4 layers you should do just 2 a day. Then after that you can do a few more per day, especially if it's sunny. It's good to sand it a bit with 100 grit between coats so it mixes up instead of creating layers. After about 8 coats you should be fine. Then you have to wait at least 24 hours before putting it in water. If the shellac comes in contact with water before it dried it will turn white. But do not fret if this happens. All you have to do is wait and let it dry completely then apply another layer of shellac and the white will disappear like magic.
What's good about it
When you apply a new layer of shellac it will merge with the old one. So if you get a scratch on the canoe you can just put one stroke of shellac and it will be gone. If you get a rip in the canvas all you have to do is put on a piece of fabric, or even better, leukoplast, and then just put a few coats of shellac. Duct tape attaches too and works in emergencies, but it won't take on the shellac.
Over time the shellac will crack up a bit (though not take in water). But all you have to do then is apply another coat. Even if you aply shellac to just a small part of the canoe it merges seamlessly with the old one so there is no need to do the whole thing every time.
Oil colors attaches nicely to both the tar and canvas.
It's lighter than a filler-coat and completely decomposable.
Here's a photo of my canoe, coated with tar and shellac.
The name comes from the worlds oldest tree, Old Tjikko, a spruce close to where I built the canoe (the canoe is also made of spruce).