Learning how to stretch canvas will give you a little more versatility in choosing your sizes and proportions for a painting. I have outlined below, how I go about stretching canvas.
Choose the size of commercial stretcher bars that you will be using. Put these together making sure that the raised edge on each matches the others. Tap the corners in tight, and make sure they are square. You can use either a square, or you can measure the diagonals. If you measure the diagonals, you need to make sure they are the same. I prefer the square. This should be done on a flat surface. Double check for squareness and insert two staples across the back joints in each of the corners. Do Not tap them down solid, as you will be removing these later. These are to keep your stretcher frame square while you apply the canvas.
Place your canvas or linen material, painting surface down on a flat surface. Place the frame with the raised edges down on the canvas. One side needs to be parallel with the edge of the material. When cutting your material, you should allow at least three inches extra, all the way around. For instance the material for an 18×24 inch piece should be cut 24×30 inches. Mark and cut the material. If you are going to wrap around and staple to the back of the frame, you’ll probably want to add a couple inches to the measurements of your material.
Find the center of each side of the frame and mark. Do the same with your material. Position the frame with the raised edges down, in the center of the material. Then place a staple in the center of one side. Pull the material tight to the opposite side and place a staple in the center of it. Do the same with the other two sides. Now, pull and staple, the alternating sides, working towards the corners as you go.
Canvas Stretching Pliers
Canvas stretching pliers are a handy item to have when stretching your canvas or linen. These have a gropping head that is about two to four inches wide. They will insure a good grip on the material and a good stretch of the material.
When you get near the corners, you can remove the temporary staples that you inserted in the back corners. Fold the material over the corners and staple. Trim off the excess canvas.
They’re those little triangular pieces that get inserted to the inside of each corner. You may not need these right away, if you have stretched the canvas well. However, you can insert them into the corners. Or you can keep them in case the canvas slackens. I prefer to place them in the corners. Then I know where they are. Make sure you give them a light tap so they will not fall out. Generally the keys are used later, if you have a sagging canvas.