In the beginning I bought the canvas by the metre (usually unprimed). I put the canvases on the stretcher, then I used gesso as a primer in multiple layers. The problem was, even if there were three layers, the canvas let the primer through, and so I had to add even more layers. And it’s not the easiest to put the canvas on the stretcher either. Nowadays I only use stretched, primed canvases, it’s much more practical. It’s important to check how many times the surface is primed, though. I tried out two high quality canvases last time. The difference was, one was primed double, the other triple. At first look, the double primed painting was fine, but once I put it under light I noticed that I could see small spots where the light shone through. This is why I primed these canvases with one more layer. The triple primed ones ended up perfect, no light shines through, the covering is perfect. For those who use thicker layers of paint it might not be an issue though.