Free surfaces in barrels or tanks allow air-oxygen to dissolve in wine at rates thousands of times greater than the rate it diffuses through wooden barrel walls or Flextank Maturation vessels. Even with CO2 in the head-space, there is usually still enough oxygen present, (CO2 dilutes air rather then displacing it), to cause aldehydic wine near the surface and to enable aerobic organisms to multiply on the surface, causing scum.

"Skins" are inert, flexible, surface-oxygen blocking disks which can be easily rolled up to load through manhole or side door. Once in the tank, they spring open to float on the wine and block the free wine surface. Their use effectively converts a fixed-capacity tank to a variable-capacity one, by greatly reducing the free wine surface-area in contact with the ullaged head space, i.e. to less than when the tank is full. This suppresses scum and bacteria formation to "safe" levels, also greatly reducing the rate of development of volatle acidity.