What is better to have full or in any position the?

The foam is slightly evaporable, if the tank is not full at all, said vapors just rusting tank wall, said oxidation is slow, because inside the tank is painted in epoxy, therefore and for greater longevity of the foam, in order to reduce as much as possible to the surface in contact with the atmosphere, an atmospheric tank containing foam is much better that it is always fully booked.