The most likely cause of warping in a canvas picture is warping of the wood frame, or "stretcher" intended to hold the painting in shape. This is usually the result of poor environmental conditions. Both painting and stretcher react to humidity and temperature. They swell in moist air and shrink in hot, dry conditions. Rooms where temperature and humidity fluctuate are especially detrimental, resulting in warping and cracking. Occasionally the canvas is stretched -- usually due to other objects pushing against it in storage.
Cover a large, well-lit work area with acid-free paper. This could be a clean workbench or an area of the floor.
Lay the painting facedown and remove it from the frame.
Examine the stretcher, assessing whether it needs repair or replacement.
Minor stretcher repairs
Use wood glue and staples to repair loose joints. Clamp the stretcher at the corners, providing padding between the clamp and the canvas.
Place a clean board across the back of the repaired stretcher. Place heavy items on this, weighting the stretcher flat. Allow the glue to dry.
Use the staple gun to reattach sections of canvas that have become detached. Ensure that the canvas is taut.
Repairing severe warping
Use pliers to remove the fastenings securing the canvas.
Peel the canvas away from the stretcher.
Lift away the stretcher.
Place the dented canvas facedown on acid-free paper.
Mist the reverse side with distilled water. Allow the water to soften the canvas fibres and paint.
Iron the canvas from the reverse with a warm iron, gradually easing the softened fibres flat.
Cut a patch of new canvas just large enough to completely cover the tear. For small tears, especially on older paintings, make patches from acid-free tissue paper.
Adhere patches to the reverse with water-soluble adhesive or with beeswax and dammar resin, available from specialist art suppliers.
Turn the painting over to the front side.
Ensure that loose canvas strands are not tucked into the mend -- use a cocktail stick to gently align the fibres. Wipe off the excess adhesive and allow the canvas to dry.
Use a new stretcher with a pre-attached fabric lining. Place the stretcher facedown on the reverse of the restored canvas.
Beginning on one of the longest sides, pull the canvas up around the stretcher. Use your hands or canvas-stretching pliers. Secure this side to the back of the stretcher with one central staple.
Repeat on the opposite side, pulling the canvas smooth and tight.
Repeat with remaining sides. Your canvas should be taut with no sags or wrinkles.
Staple the canvas around the stretcher at 5 cm (2-inch) intervals, pulling it tight and tucking it under at corners.
Attach large or valuable canvases to temporary stretchers during repair. Avoid hanging restored painting above sources of heat or moisture. Have the painting valued. A valuer may advise professional restoration. With antique works, consult a trained conservator.
D.I.Y. repairs are only appropriate on inexpensive or amateur paintings. Old canvases can be very brittle. Never attempt to fix paintings so warped that the paint is cracking. This is a job for a professional restorer.