Sometimes the cloth was stretched over the wood panel, but normally cloth is stretched over a rigid wood frame, commonly known as stretcher bars. Cross braces can be used on large frames to prevent bowing as the cloth is stretched tight. To reduce the effects of acid outgassing from the wooden stretcher bars it may be advisable to coat the stretchers with acrylic gesso or polyeurethane and allow to dry before stretching the canvas. If the canvas becomes loose on the stretcher frame, wooden or plastic keys in the corners can be hammered into place to spread the stretchers and tighten the cloth. Best offers a unique metal key than is very stable and rust proof and is easily adjustable.
Some manufacturers add anti-mold treatments to their preprimed canvas.
As a tanner it is said to protect cloth from deterioration. However, in discussions with a conservator, she did not recommend the use of formaldehyde because it becomes brittle. The best protection for the back of canvases may be to loosely tack a piece of archival board to the back of the stretchers. This allows air circulation and escape of humidity while protecting the cloth from deterioration. Older canvases have been found to decay least where they are covered in back by the stretchers.