Stretching your own canvas is fairly easy once you know the procedure and have the proper tools and materials. Note that the canvas referred to below is pre-primed; but you could also use the following procedure with raw canvas and, after it has been stretched, prime it with two coats of acrylic gesso. Please read through all of the instructions first, since certain steps must be prepared for in advance.
First you will need to gather a few tools and materials: canvas pliers, stapler/staples or hammer and tacks, stretcher strips, and canvas. In addition, a T-square is convenient to help you insure that the stretchers are square. Then follow these steps to stretch your canvas:
Select the appropriate-sized stretchers for the canvas you want to build. Your art supply store has a variety of pre-cut wooden frame rails (or stretcher bars) that have interlocking mitered edges.
Once your stretcher frame is assembled on a flat surface, you should verify that it is square. Do this by taking a tape measure and checking that the diagonal distances from both opposing corners are equidistant. (They almost always will be.) If you encounter an offset, you can correct it by attaching a small metal right-angle brace on the inner frame corner. A small plywood right-angle triangle attached to the rear frame corner will also help correct any shift.
For frame rails longer than 36 inches, an interior brace should be inserted between these rails to support them from bowing inward after the canvas has been attached. This will also protect against frame distortion during periods of fluctuating humidity. If this type of support rail did not already come with your pre-cut frame, you should cut a piece of similar wood and attach it with the type of braces detailed in step 2.
Take the canvas and lay the primed side down on a clean, flat surface. Place the frame atop the canvas and then cut away any excess material so that there are two inches of excess canvas bordering the frame on all sides. It is important that you leave this border material because this is what you will use to grip, stretch, and attach the canvas to the frame.
Fold one side of the canvas over one of the shorter frame rails and then attach a canvas tack at the center of the outside edge of that rail.
At the opposite side, use canvas pliers (which are similar to regular pliers except they have a broader gripping area) to grip the canvas at mid-rail. It may be helpful for you to set the frame upright while doing this. With a firm grip, pull the canvas until a straight crease is formed to the tacked end. While keeping tension on the canvas, insert another tack at the center edge of the rail, just like the other side. Note that you may also use a staple gun/staples instead of tacks. (The Arrow JT-21 is easy to use.)
Move to the next frame rail (one of the longer ones) and repeat steps 5 and 6. As you do, you will notice a triangular canvas crease as you attach the third side and then a diamond-shaped crease when the fourth tack is attached on the remaining mid-rail.
Now, place temporary tacks at all four corners. Starting with the center of one of the long rails, grip the canvas tightly with the pliers and fasten tacks at 2″ intervals. Repeat with several tacks in both directions (from center) and then switch to the opposite side and repeat this process. Continue working from the center until both long sides are completely tacked down to all but one inch from each corner, stretching the canvas evenly as you proceed.
Once both long sides are finished, remove the temporary tacks that you fastened in step 8. Now repeat the same fastening process for both of the short sides, working out from the centers. For smaller canvases, one entire short side can be fastened first, followed by the opposite side. Larger canvases should be rotated several times to evenly stretch the material over the frame. Note that a primed linen canvas (oil-primed linen, in particular) will normally require that the tacks be placed closer together, due to the limited stretch of this material.
Fold and pleat the corners of the canvas and then neatly wrap them around to the rear of the canvas frame. Keeping tension on the material, tack all of the excess cloth to the rear of the frame so that it is neatly secured. Then place tacks on all four outer corners. This excess material is important to have in case you ever need to restretch or remount the canvas.
Any remaining canvas indentations or ripples can be reduced by moistening the affected area with a damp sponge. Be careful not to saturate, and allow the canvas to dry completely in a well-ventilated area.
Keep in mind that it is easier to stretch primed cotton and Polyflax canvas than it is to mount a primed linen canvas, since linen fibers do not stretch as much. Oil-primed canvas requires the most attention when mounting on a frame because it has virtually no stretch. When working with this type of canvas, you may have to remove tacks in wrinkled sections and then restretch and retack.
By following the above steps and using proper materials and a few tools, you can easily build your own canvas. With a little practice, the process becomes much faster and easier, almost to the point of becoming routine.
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