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: