Pictures that are painted with casein colors are exposed to the vapors of formaldehyde, which treatment renders them insoluble in water. But inasmuch as this treatment is impracticable for large surfaces the practice is to add a certain percentage of formaldehyde to the ready for use paint immediately before application. To heighten the efficiency of the paint in this respect it is recommended to add to the water used for washing or cleaning of the surface, a small portion of solution of formaldehyde.
While casein cold water paints are much more economical in first cost and on account of the disinfectants introduced really more sanitary for the walls and ceilings of hospitals and other public institutions, it would be fallacy to expect that they are equal in wear and durability to enamel paints or varnish. But for the purpose mentioned it must be said in favor of water paint that after its drying there is still a certain degree of circulation of air that is not the case where enamel or varnish is used.
When casein paints are used, be it for interior or exterior work, it is an absolute necessity that the surface is dry and that there will be no moisture before or after applying the paint. Ordinarily the surface should be somewhat porous, as the paint will not cover well or hold well on very smooth, glossy surfaces. Hence any kind of masonry, be it stone, brick, terra cotta, concrete or cement, also wooden girders, joists, in fact, any sort of timber may be successfully coated and made to some extent fire resisting. Lately casein paints have been placed on the market that contain certain percentages of linseed oil, and are recommended for painting iron and steel, claiming for them rust preventing properties. The makers, however, are very careful to direct that the surface must be entirely free from rust before it is coated, a caution that holds good for any iron and steel protective paint.