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. Notes Silica can be purchased at pottery supplies under the brand name Silex.
Silex dust can cause breathing problems, so please always use a respirator when sanding filler.
Lead is known to cause brain damage when absorbed through the skin or inhaled as dust. Be very cautious using and disposing of white lead in your filler.