Restoration Fabrics are specialist fabric used for re-lining paintings. This gives the original canvas support and helps prolong its life.
Some artists like to pre-stretch their new canvases with this material to give a more rigid painting surface.
Polyester Restoration Fabric is an un-dyed woven heavy weight polyester sailcloth that has been scoured and heat set. It has great dimensional stability and provides an air-tight barrier against the back of the canvas surface.
Polyflax Lining Canvas is a similar material, though wider and less densely woven. It has great dimensional stability and has been successfully tested by the Courtauld Institute.