Canvas filler formulas have been guarded for decades by wood canvas canoe builders all over the world. The formulas below have been published or made available in a legal manner and not "stolen" or otherwise "borrowed" without permission. If you have another formula that is not listed here, please send it in!
One note about filler formulas. The materials that were used in the early 1900's may not be the same as materials with the same names today. In addition, canvas is certainly different today than it was in 1900, so some of these formulas may not provide the best coverage for your money.
Commercially prepared formulas are available from builders in the Online Builders & Suppliers Directory.
I can suggest a recipe, the best I remember it from 20 years ago. It was based on glaziers putty and floor varnish - we used Hippo Oil brand at the time. Glaziers putty is basically clay and linseed oil. We warmed the varnish and mixed (kneaded) the putty into it by hand. I honestly don't remember the proportions, but we ended up with something like a thick pancake batter. To this we would add some japan drier to accelerate drying. This mixture was worked into the nap of the canvas by hand, in thin coats. If allowed to dry between coats, it wouldn't build up into a single soft thick layer. It would remain flexible, and as many layers were applied as were required to fill the canvas. Two coats of orange shellac with light sanding between were applied over it prior to painting with enamel paint. This recipe originated from someone in the Dwight, Ontario area, who was generous enough to teach a few of us to repair and re-canvas the fleet of Chestnut canoes we battered on the rocks of Algonquin park.
More from Dom Williams: I used your site to prepare a filler based on the floor varnish/glaziers putty/indian dryers mixture listed in the site; the author could not remember proportions. 0thers using this formulation may be surprised to find how much putty is required versus varnish. I wound up with a mix of 1cup varnish/ 2 1/2 lb putty and 1 tablespoon of dryers and probably would have been better to increase the putty to 3lb. To refinish a 16 ft canoe with the existing filler largely worn away by use and/or paintstripping I used 4 batches ie 1 quart of varnish and 10lb of putty; the final batch was not all used. I found it applied best using a cheap 8 inch plastic drywall knife (the more flexible the better) and applied it from the gunwales up and then from the centerline to meet the "upstroke". I "spot-primed " the areas where the old filler had largely washed out of the canvas by hand rubbing glops into the weave before doing the overall trowelling.