Most billboard posters are printed on a high quality, 120gsm blue backed paper that is suitable for paste application. This heavier than standard weight offers greater tear resistance and easier handling and is blue backed for reduced show through of over pasted posters.
When an advertisement expires and there is no demand for its replacement, a crew of installers is responsible for covering it with the back side of leftover posters. This procedure ensures that no advertisement remains on display beyond the agreed date.
During a cloudy winter day all billboards in Milan were covered with the backside of the billboard poster paper Blue Back. The blue color of the inverted paper corresponds to the blue of the sunny sky and thus during this operation the sky was at the same time sunny and cloudy; in a city where the numerous billboards are able to create the perception of a second sky under the real one.