I ran a series of tests on #10 cotton duck using wood stains and artist’s oil paint covered with spar varnish. I put on 2 or 3 coats of varnish. A week later I took a 3 “D” cell Maglite and whacked the canvas. It took very little effort to split right through the canvas. I was totally bummed out and more than freaked out

The spar varnish must have been too thick and in essnce weakend the canvas by making it too stiff.

Roger