A size seals the porous fabric and isolates it from the ground and/or oil paints. Linen and cotton will prematurely rot without a size layer.
Diluted with distilled water, PVA size is a contemporary size for fabric support. On linen and canvas, it will not re-absorb atmospheric moisture that eventually causes swelling and shrinking and ultimately, paint cracking. It retains its flexibility. It has neutral pH means that it will not yellow.
