Claim: Lining a painting, sometimes referred to as relining, flattens the surface of the painting or strengthens the painting by attaching a new canvas to the back of the existing one.
Evidence: The lining of paintings is a process of restoration used to strengthen, flatten or consolidate oil or tempera paintings on canvas by attaching a new canvas to the back of the existing one. The process is sometimes referred to as relining. 
The purpose of lining a painting is to flatten or consolidate its surface or strengthen its support by attaching a new canvas to the back of the existing one. The process has sometimes been referred to as relining.