Abstract: The invention concerns a film-forming substance for temporarily opacifying transparent or translucent surfaces made of glass or rigid or flexible plastic, such as, for instance, agricultural or horticultural greenhouse covering, characterized in that it contains, in slightly alkaline water: at least one binding agent consisting of a resin based on acrylic derivatives, having an acid index higher than 60; and at least a filler or pigment selected, for example, among the following; silica or silicate; barium sulphate; titanium oxide; zinc oxide; mineral or organic colored pigment.