Painting by Immersion and by Compressed Air: A Practical Handbook

by Arthur Seymour Jennings

The following article by M. C. Hillick appeared in "The Painters' Magazine," of New York, and will doubtless be read with interest:—

The Willys-Overland automobile factory has recently been installed with sixteen large enamelling furnaces or ovens having a volume of 48,000 cubic feet and a capacity of 140 tons of enamelled product every ten hours. These ovens have been electrified and they require approximately 5,500 horse-power. For some months past the Overland Company has been testing one of these electrically-heated ovens, and the results have, in every way, measured up to expectations. Formerly the company, in common with practically all other companies using enamelling or baking ovens, employed gas as the treating medium. The ovens now electrified are almost entirely automatic in operation. When the oven is loaded the closing of its doors automatically throws a switch which turns on the current. A pyrometer which can be adjusted to operate at any desired temperature rings a bell when the proper degree of heat is reached, thus notifying the attendant, and also automatically turning off the current. The electrically heated oven does away with all flue gases and with their attendant dirt and spots. It also reduces the required volume of ventilation to the minimum, thereby eliminating air currents and the dust which, in greater or less degree, usually accompany them. The electrification of the ovens does away with explosions, banishes danger from fire, and gives an assured "safety first" to the workmen. The heat is said to be non-oxidizing and, therefore, cannot scald the operator. The working quarters are rendered comfortable and a higher grade of work is made possible.

Fig. 124.—Show Card Done by Spraying (Strong).

While for the custom shop painter it is a long way to such equipment, the fact that these facilities are gradually being acquired leads to the assumption that, in the course of a few years at most, some portion of the work coming to the shop for painting repairs will be handled through the baking oven. In connection with this subject of enamelling and oven baking, the various costs of paint and varnish, it is to be noted that the excessive temperature employed in baking is being criticised as detrimental to the finish. Recent tests are said to have been made which show that baking paint and varnish at the maximum temperature shortens the life of the finish. Mr. J. W. Lawrie, of the Chemical Works of Milwaukee, Wis., has stated that, as a rule, the lower the temperature and the longer the time the paint and varnish is baked, the finer the appearance, service and durability of the finish and the greater its capacity for resisting moisture. The finish will have more elasticity and deeper lustre. Mr. Lawrie is of the opinion that twelve hours at 180 degrees are better than five hours at 280 degrees.

In some motor car shops, within the past two years, by the use of baking ovens cars were painted and finished, all coats being baked, and made ready for service in three days. However, we are not commending or recommending this practice. For the custom shop painter, especially, it is entirely unsuited. Nor would we recommend the baking process for surfaces other than metal. The same disadvantages marshalled in opposition to baking paint and varnish upon wood surfaces fifteen years ago, or longer, still remain in evidence. For aluminium, sheet steel or other metal panels or metal surfaces in general, the baking process offers an opportunity for finishing work under conditions more uniform than any which may be expected to prevail outside the oven. An other advantage is noted in favour of oven baking. It permits the use of more elastic materials without the aid of artificial oxidizing agents. As compared to the present air-drying system, the oven baking method, operated upon a conservative basis, permits the car to be thoroughly painted and finished in six or eight days. The use of more elastic materials—paints, colours and varnishes—has been mentioned as a part of the oven baking method. Upon steel surfaces this is perhaps more necessary than upon aluminium, and possibly iron. Steel has a linear expansion double that of wood. As a matter of fact, experts in these matters employed by the Pennsylvania Railroad assert that the contraction and expansion of steel surfaces is much more pronounced than the same action in wood. Materials of greater elasticity as compared to those used in natural air-drying practice are urged.

Primers and surfacers, and the general class of foundation coats, will require an average of three hours' baking at 200 degrees F. While some colours require higher degrees of heat than others, 170 degrees F. baked for, say, six hours will dry the average colour, excluding white. The latter pigment, baked at a temperature varying from 85 to 110 degrees F., will dry properly in the course of a few hours, and retain its natural purity of colour, whereas at a higher degree of heat the white takes on an objectionable yellowish cast. Black, at the opposite end of the colour pole, can be safely baked for six hours, at something like 200 degrees F. Finishing varnishes, taking them as they run, will bake at from 110 to 150 degrees F. for five or six hours. In all baking practice the personal equation figures largely. Reason, good judgment, the capacity for taking pains—all these are items of importance.

Oven baking methods are being successfully employed in some of the large city repainting establishments. A firm near New York, for example, using what is known as the radio process, paints and finishes a car in three days. The cleaning of the cars is accomplished by the use of a steam jet, a treatment which is said to cut away the grease like magic. All surface defects following the cleaning of the car are touched up and faced over with the necessary filling and surfacing materials. Then these patched-up parts are rubbed down with water and rubbing brick, and the general surface of the car is lightly rubbed with pulverized pumice stone and water. All colour, and varnish colour, coats are applied with a paint atomizer. This atomizer is a pistol-shaped device operated with a trigger, the material being sprayed from the muzzle of the barrel. The varnish colour is baked for three hours at a temperature of from 110 to 120 degrees. From 90 to 100 degrees of humidity are provided for the oven, and by means of an exhaust fan a fresh supply of air is furnished every three minutes. In the oven where the varnish colour coats are baked a thermostat is installed, which regulates the temperature. All the air entering the oven is washed and purified by running it through a water tank before it enters the oven. This water-washed air is forced into the oven by a fan blower, and contact with a radiator superheats it.

In practice, the high humidity here referred to, and the water-washed air, are mediums which serve to keep the outer surface of the drying coat moist while the inner surface is drying, in this manner furnishing in due time a paint film dried uniformly from top to bottom.

The finishing varnish is dried in an oven having a maintained temperature of from 90 to 100 degrees, the humidity being regulated at from 60 to 70 percent. This humidity is likewise found to assist a varnish film to dry uniformly throughout.