I prepare my own canvases; the top left picture shows the stretcher bars and my first four assembled. I cut my 12oz cotton duck to size with a good fold over on each size, then I start with a staple in the mid bar on one side. I turn the whole through 180 deg. stretch the canvas and then staple in the middle of the opposite side, I then turn through 90 deg, stretch and staple the middle. Turn 180 deg. and staple the middle. Next stretching as I go I staple along the sides leaving the corners free. I cut across the corners an inch out, fold one side in and then fold the other side of the corners see bottom left.. Next I put hte canvas flat down and paint acrylic / gesso on the back and sides. Turn it over using a cork cut into 4 to balance the underside and paint the flat side. This will take up to 12 hours to dry. I then pint the whole again. 2 coats are generally sufficient but you can use 3. Lastly I rub a base colour on the canvas – I like burnt sienna or brown madder plus white spirit (odourless).