Public demand for more durability in automobile finishes has led to new developments in finishing materials and methods through cooperation of finishing materials manufacturers and automobile builders. By experimentation it has been found that certain cellulose nitrate materials, when applied over suitable under-coats, dry quickly in the air by evaporation of the solvents and leave a film that is hard and tough. Its durability is many times greater than that of the most durable finishing-varnish and, as it has been discovered that sufficient luster can be produced by rubbing and polishing the unprotected cellulose-nitrate surface, one of the large automobile production plants adopted, in July, 1923, as its standard method of finishing, the use of such a finishing coat over primer and surfacer coats, obtaining the luster by polishing the cellulose-nitrate top-coat. A number of companies have now adopted this process.
Certain finishing-material companies, having discovered how to increase the quantity of cellulose-nitrate in solution without increasing the viscosity, made possible its use for automobile finishing.
The change from a varnish to a pyroxylin finishing-system seems simple but a cellulose-nitrate finishing-material is complex. A large number of gums and oils can be used to obtain the desired properties but considerable study is required to determine the kinds and the quantities to use. The softeners and the stabilizers may be part of the solvents but they have their own duties to perform. The solvents hold the cellulose nitrate in solution and their evaporation permits the film to be deposited and harden. The nature of the solvents greatly influences the character of the film. Not all pyroxylin materials have great durability. This is obtained only by using the proper basic materials, combined in the right proportion and applied in the correct way.
The fundamental cause of the failure of finishes is the contraction of the top material while the underlying material is still moist. During exposure to rain or other dampness out of doors, the entire film absorbs moisture, swells and softens. When the sun comes out, the outer surface dries and contracts and, if the material is not sufficiently elastic to withstand the stresses, cracks develop. Durability can be increased by reducing the severity of these factors. By nature, the pyroxylin film is much less absorbent than the varnish film and the particles of pigment shade the materials beneath so that only the extreme outer surface is exposed to the sunlight. In time, the moisture and sunlight do, however, cause a type of failure known as “chalking.” This effect is of such microscopic depth that it can be removed easily by washing with a mild abrasive or by polishing, thereby exposing a fresh surface, and in the case of a good cellulose-nitrate finish the washing or polishing can be repeated many times before any serious wearing away of the surface occurs. By mild polishing every few weeks, together with ordinary cleaning of the car, the finish can be kept with an appearance as good as it had on the showroom floor.
The author describes the fundamental operations in finishing an automobile body with paints and varnishes, explaining the purposes of the different coatings and the filling and rubbing, and classifies the successive coatings in all finishing systems as (a) primer, (b) surfacer, (c) color and rubbing, and (d) finishing. To determine the cause of the rapid failure of finishes, test panels were exposed to the weather during different periods of the year and it was found that the length of life of varnishes depends greatly upon the season, varying with the amount of sunlight and heat, being shortest in summer and longest in winter. These tests also showed that the primer and surfacer coats were very durable but that the color and rubbing varnishes were of low durability, failing in from 1 to 5 weeks when unprotected by finishing varnish.
The real reason for the short life of the ordinary automobile-finish was discovered to be the failure of the color and rubbing varnishes, which crack and take the finishing varnish with them. However, the finishing varnish is itself partially responsible for the failure of the color and the rubbing coats, because its film is easily scratched by grit and cleaning or by other mechanical injury, and failure of the underlying coat starts at these unprotected spots. The life of a varnish surface is shortened by regular polishing even if it is washed regularly only with water. A polish that is as nearly harmless as any is made of linseed oil mixed with enough turpentine to prevent the application of too heavy a film of oil. The life of ordinary production varnish-finishes can be doubled or quadrupled by the use of color and rubbing varnishes of high durability but they require a long time for air-drying and forced drying is likely to injure the color.