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.
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.
When it is stated that a pistol sprayer consumes say two cubic ft. of
free air per minute, it means when working almost continuously, but as
the periods of actual work do not represent more than two-thirds of the
actual number of working hours, there is a margin left. Nevertheless,
it would be unsafe to state a lower figure, for in some cases where
convenient feeding of the work to the operator is arranged, the
consumption of air is continuous.
It should also be remembered that the air is used for other purposes,
i.e., a small amount is used to provide a pressure feed of colour to
the instrument in many cases, and where a heater is employed, a small
amount of air is bypassed through the pistol to keep this warm when
spraying is momentarily stopped. Although the practice of heating the
air slightly increases the ultimate efficiency of a given compressor,
yet it is unwise to count too much upon this fact.
In submitting the following figures as to air consumption, the
Airostyle pistol has been taken as a type:--
For work with a tip and needle of 1 m/m. dia., 1 cub. ft. per minute;
1½ m/m. dia., 1¾ cub. ft.; 2 m/m. dia., 2½ cub. ft.; 2½ m/m. dia., 3
cub. ft.; 2½-3 m/m. dia., 3½-4 cub. ft.; 4½ m/m. dia., 5 cub. ft.
These consumptions naturally vary slightly with the adjustment of the
nozzle of the pistol, but they are from actual tests and so may be
taken as authoritative.