I filled the canvas today. The filler is a mixture of silica powder, linseed oil and other ingredients that makes the canvas water tight. The first coat took about an hour and a half, working the mixture into the weave with a stubby paintbrush. The second and third coats go a lot faster. Between coats, the finish is rubbed smooth with a mitt made of scrap canvas. All told, it was about 4 hours of scrubbing the filler into the weave and rubbing it smooth. My arms are going to be sore tomorrow. I feel like I just swam about 2 miles. This stuff takes about a month to cure, so I'll be caning the seats and varnishing in the meantime.