The advantages of the latter type are many, and in addition to being designed, in most cases, to produce the greatest effect with the least possible consumption of air, they are also capable of applying the paint in very finely divided particles to the smallest work, such as buttons, imitation jewellery, small electrical camera parts, or of applying silky coats to such large work as motor bodies, constructional iron work, large tinplate work, agricultural machinery, domestic gas apparatus, etc. at a speed approximating to four to ten times the speed of handwork, and giving a far superior finish, in many cases with less coats.
Many people who contemplate the subject of painting with a spray think only of a smother of paint being discharged from a nozzle. They are not aware that the colour can be better controlled in a suitably constructed spray than is possible with a hog's hair brush.
The flow of paint is stopped or started instantly, and the amount of paint delivered is at all times under perfect control, so that a quantity corresponding to that delivered by a ¼-inch brush or pencil can be increased to the quantity distributed by a 4-inch brush in a single stroke. In other words, you have a tool which is the equivalent of half a dozen brushes ranging from ¼ inch to 4 inch in width.
A little practice is, of course, necessary to master the instrument, but nothing like the practice which is required for successful painting with hog's hair.
The principal care is to put on the paint until the beads of paint coalesce. Too much would make the paint run, too little would not leave the surface covered. When properly done the surface is superior to other painting, as the hairs of the brush are not dragged through it to disturb the evenness of the surface.
The air pressure required for painting varies with the consistency of the paint, its viscosity as well as its thickness. Some liquids have a quality of stringiness or hanging together which require a higher pressure to break up.
Thin lacquers and varnishes may be sprayed with 18 or 20 lbs. to the square inch, and from that to about 50 lbs. will cover most classes of paint.
The lowest pressure at which a paint breaks up is the best pressure to use, as high pressures have a tendency to make more dust and put fine particles of paint in the air, where they are not wanted.
The volume of air has also to be considered. This varies with the size of the aperture through which it exhausts and to some extent with the pressure—for a small nozzle such as is used for lacquers and thin liquids, say, one cubic foot of free air per minute, and up to 3 cubic feet for oil paints.