The wooden frame of a painter's canvas may shrink over time causing the canvas to sag. To fix this problem use canvas stretcher keys. These keys are small wedges of either wood or plastic you may insert into the corners of canvas frame. The keys push the canvas bars back into position and tighten the canvas. Canvas keys are either pre-inserted into the frame or come stapled to the frame in a small plastic bag.
Lay the canvas down on a flat surface with a towel or other material underneath to protect the artwork from damage.
Insert the pointy end of the canvas keys into the slots located in the corners of the frame. Each corner will have two keys. Place the long side of one key flush against the inner part of the frame on the length side, and the other key flush against the inner part of the frame on the width side. Some canvas frames come with the keys pre-installed.
Press the keys into the space between the two sides of the frame that come together at the corner by using your hands. Try to place the same amount of pressure into each key around the frame to avoid buckling the canvas.
Check the tightness of the canvas. If the canvas is tight, no further adjustment is needed. If the canvas is still sagging place the canvas back on the towel face down.
Place a small block of wood flush against the back of the key and hit the block with the hammer. This protects the key and canvas from any hammer damage.
Continue pressing the keys into the frame until the canvas is tight.
Apply gradual pressure to the keys and check the tightness of the canvas regularly.
Do not over tighten or you will damage the canvas.
Knowing how to use canvas keys properly is crucial to the proper maintenance or valuable paintings.
If your frame did not come with keys, you can buy them separately at most frame stores. Keep a few extras handy in case the ones in your frames go missing or get damaged.