A paint film both absorbs and transmits water and water vapour. The quantity of water absorbed can be measured by weighing a coated glass panel, immersing it for a known time and reweighing it after wiping the surface dry. The percentage weight of water absorbed can be expressed.

It is appreciated that any blistering of the film should be avoided. Blisters will tend to trap water which is transmitted through the film.

The transmission of moisture through a paint film may be measured by placing a paint film between two cells, one containing high humidity, the other containing a dissicant such as calcium chloride. The amount of water vapour passing through the paint film is measured by weighing the dissicant at intervals and noting the increased weight.