Curious about Wooden Canoes
Early May, 2012: Ah, yes. The microballoons.... Well, I mixed up the filler per a common formula—boiled linseed oil, exterior alkyd enamel, thinner, a bit of varnish, and Japan dryer, but substituted the glass bubbles (microscopic indeed) for silica. Instructions were to substitute volume for volume, and that a volume of bubbles equal to volume of silica might weigh a pound and a quarter or so.

So I mixed it all up with a beater on a drill, which went fine, and ended up with a medium batter. Dipped in my brush and applied the first load to the canvas. All the liquids were instantly sucked out of the mix, leaving a damp and almost weightless paste on the surface which was impossible to work into the weave with the brush. I ended using the brush to transfer the filler to the canvas, and then mashed it into the weave as best I could with my gloved hand. In the process, quite a bit of the paste flaked off onto the floor. But I was committed, and kept at it. No turning back now.

Got around to the last quarter and wondered what would happen if I wet the canvas first. Happened to have a can of home-made oil-varnish wood finish (roughly equal parts oil, varnish and thinner) on the bench, so I brushed this on the last of the bare canvas, and then hit it with the filler. The filler stayed brushable. End of coat #1.

Kept checking on drying progress, and gave it a final glove smoothing at the right time. The area I had wetted before applying the filler took several hours longer to become workable, as you might expect.

The following day I inspected, knocked down any ridges, and applied a second coat. Brushed on well and didn't use much, since the weave was already saturated. When the second coat was dryish (but not cured, of course) I went over the whole boat with a 100-grit flexible sanding block to even out the application--get rid of ridges and rough spots--and dabbed on a bit more in a couple of places.

The hull is now hanging from the shop ceiling. No hint of the canvas texture showing. Surface looks wonderful, although it is soft/powdery to the touch, nothing like silica filler. It will hang there until mid-July when I return from Montana. Hope it will have cured good and hard!

18 July, 2012: The filler has not cured “good and hard” after nine weeks. In fact, it seems no harder than it was five weeks ago. I rolled it up on horses so I could sand the bottom, and the edges of the horses left palpable creases in the filler/canvas. Need to move ahead, so will paint and hope for the best.

I’m curious how my experience with this compare with experiences of others, and if I made some grievous error in the process.
Is there any difference in the area where you applied your "home-made oil-varnish wood finish" or is it soft all over?
Micro balloons are not the problem. I use them all the time to save a few pounds Brand of boiled linseed oil used could be the culprit, you did use boiled linseed oil? There are a couple of different driers used to produce "boiled linseed oil". You may have used one of the slower drying oils. Or did you use unboiled and then not add enough drier. That may be what has happened. The last one I did this spring was 5 weeks although it was probably hard enough at 4 weeks. That was with micro balloons. Silica is silica regardless of shape, granular or micro balloon. That doesn't have any affect on drying. Drying is the polymerization of the linseed oil and other resins in the paint.
Good observations, Jan. The can said "boiled", although it had been around the shop for many years and other oils may have migrated into it.... I'm guessing that short of recanvassing, the only option at this point is to forge ahead and paint. What are your thoughts?

What has your experience with application (not the curing) of this filler? Similar to mine?
Before you try re-canvassing, how about a test of gentle heat to see if that helps the consistency of the filler?
Could give that a shot. Actually, I have no intention of recanvassing.... Paint and move on!!!!!!!!!