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
Varnish is more than simply a layer to protect your oil painting from ultraviolet light, pollution in the atmosphere and abrasion. It will also bring out the brilliance in colors, making them sparkle. Varnish is made from Damar resin dissolved in a mixture of turpentine and other organic solvents. Keep in mind that oil paint dries from the outside in; when it dries, it forms a surface skin first. During the drying process small pores form in this surface skin. The surface of the paint may feel dry, but under the skin, the paint is still wet, no matter how thin the coating is. It takes at least 6 months drying time for the dried oil paint film to become dry enough that it won't be re-dissolved by mild solvents such as turpentine. When a varnish is applied to a reproduction art oil-on-canvas painting before the 6 month drying time, the varnish fills the pores of the oil paint film, The solvent content of the varnish can easily re-dissolve the incompletely dried oil paint at the bottom of the canvas and cause all kinds of problems - such as wrinkling of the oil paint film, cracking, and yellowing of the color.
- Clean the painting so it's free from dust and dirt. Lay the oil painting flat, then dampen the surface a bit of cotton wool with clean water.
- Dry the oil painting with another bit of cotton wool. With your fingers, gently remove any cotton fibers that have been caught in the oil paint.
- Leave your painting to dry for several hours.
- Choose from high gloss, satin or matte finishes. Varnishes may be brushed or sprayed on. Use a flat bristle brush to apply the varnish. If you don't want your oil-on-canvas painting to be too shiny, use a matt varnish rather than a gloss one.
- With the painting flat, work from the top to the bottom, applying the varnish in parallel strokes from one edge of the oil painting to the other. Always work in the same direction. Try to have the same amount of varnish on the brush for each stroke so you put equal amounts of varnish on all parts of the oil painting. Always varnish the whole of the oil-on-canvas painting in one go.
- When the first coat of varnish is dry, apply a second coat at right angles to the first. This will give you a good, even finish.
- Leave the painting flat for at least 10 minutes after you've finished varnishing to stop the varnish running down the oil painting. Then prop it up against a wall to dry, face inwards.
- To test whether the varnish is dry or not, touch the edge of the oil painting to see if it's still tacky. It should dry within a day or two, depending on the weather.
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