Oil paintings on canvas are always mounted on inner, wooden frame bars (a stretcher) before framing. A stretcher is a wooden support that a canvas is attached to for stability. A frame around the stretcher, in addition to complementing the appearance of the painting, provides extra support for the canvas.
A competent picture framer will be able to stretch the oil painting onto a stretcher frame or you can stretch the canvas yourself fairly easily once you know what to do and have the right tools and materials.
Making a Stretcher Frame
You need the necessary tools and materials: a stapler, stretcher bars, and possibly also a pair of canvas pliers to grip the canvas (similar to ordinary pliers except they have a broader gripping area). You can buy stretcher bars and staplers from art supply shop.
Measure the size of the painted area that you want on display and cut the stretcher bars to size. Then assemble the stretcher frame using the bars, pushing the mitred ends into each other and gluing the joints. One or more cross bars may be needed depending on the size of the oil painting and the strength and stiffness of the stretcher bars.
Check that the stretcher frame is square, either by using a T-square or by taking a tape measure and checking that the diagonal distances from opposing corners are equal. If these are equal, the frame is square.
Stretching an Oil Painting Canvas onto a Stretcher Frame
To stretch your oil painting onto its stretcher frame, follow these steps:
• Unroll the canvas and lay the oil painted side down on a clean, flat surface. Place the frame on top of the canvas. It is important to leave any excess overlapping material because this is what you use to grip, stretch, and attach the canvas to the frame.
• Fold one side of the canvas over one of the shorter stretcher bars and then attach a staple at the centre of the outside edge of that bar.
• On the opposite side, use pliers to grip the canvas at mid-bar. With a firm grip, pull the canvas until a straight crease is formed to the tacked end. Insert another staple at the centre edge of the bar, just like the other side.
• Move to the next stretcher bar (one of the longer ones) and repeat steps the previous steps.
• Place temporary staples at all four corners. Starting with the centre of one of the long bars, grip the canvas tightly with the pliers and staple at 5 centimeters (2 inch) intervals. Repeat with several staples in both directions (from the centre) and then switch to the opposite side and repeat the process.
• Repeat the same fastening process for both of the short sides, working out from the centers.
• 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, staple all of the excess cloth to the rear of the frame so that it is neatly secured. Then staple all four outer corners. This excess material is important to have in case you ever need to re-stretch or remount the canvas.
Re-stretching an Old Oil Painted Canvas
Changes in climate will cause the canvas to expand or shrink, resulting in a sagging or rippled canvas. The oil painting will then need to be re-stretched onto a new stretcher if the problem is serious or a less serious sagging can be remedied by either altering the tension provided by the corner wedges in the stretcher or by applying a special spay, available in art shops, that tightens the canvas