PAINTING BY COMPRESSED AIR.
The Evolution of the Apparatus Employed.
By way of introduction to the subject of the application of paints, enamels, lacquers, varnishes, stains, etc., by means of compressed air, a few notes on the development of the method may be given, if only with the object of correcting the impression, which sometimes exists, that such apparatus as is now employed is more or less of an experimental character.
The early attempts to apply paint, etc., to articles other than by the old method of brushing were mainly carried out with a type of machine still largely used for whitewashing, and the principle adopted was to partly fill a convenient tank with the paint or whitewash and to pump a pressure of 35 to 40 lbs. into the tank by means of a suitable hand pump. The paint was forced through a flexible tube attached to a connection near the bottom of the tank, and at the other end of the tube was a simple form of nozzle with a trigger control for the paint.
It may be said at once that this method was doomed to failure owing to the underlying principle being at fault. While eminently suitable for applying whitewash or distemper, it was hopeless for paint of a more viscous character, owing to the fact that the latter was not atomised as it issued from the nozzle.
The type of machines referred to, viz., those which are used for spraying lime white, distemper, whitewash, etc., are fully described and illustrated in another chapter.
After failing to achieve satisfactory results with such simple apparatus when applying viscous paints, etc., further experiments were carried out with a somewhat different type, commonly known as the "kettle" type of sprayer. The principle employed was to employ compressed air, supplied, in this case, not from a hand pump but some form of power compressor, and at a pressure varying from 20 to 50 lbs. per square inch.
The air was delivered through a nozzle controlled by a convenient plunger or trigger type of valve, and impinged at an angle across a similar nozzle communicating with the paint receptacle of the sprayer. The action of the air impinging on or across the paint nozzle created a vacuum in the paint tube, and thereby sucked up the paint from the receptacle and gave a fan-shaped spray. Up to a point these experiments proved satisfactory, but still did not overcome the difficulty of successfully dealing with intricate shapes, small articles, and particularly in applying heavy paints with fair percentages of varnish embodied therein.
The Concentric Form of Spray.
Attention was therefore directed to the concentric jet form of sprayer, and this is now almost universally employed where painting, enamelling, etc., is carried out on a large scale.