SPRAYING VERSUS BRUSHING.
It will be convenient now to consider the length of time involved in
spraying, as compared with that involved when painters' brushes are
used.
In dealing with this question, it should be pointed out that while some
little gain is apparent in the actual time occupied in handling the
article, which is mostly done on turn-tables when spraying, yet no note
is taken here of the difference in handling, but only the time occupied
in covering the article with paint, japan, etc.
We will take, by way of example, a modern gas fire. Very few of these
are now brushed. It is the consensus of opinion that a gas fire takes
10 minutes to brush.
To spray one, with a superior finish, takes 30 to 45 seconds, and even
where some difficulty is experienced in handling, or where it is the
practice to have the firebricks in place beforehand, so necessitating
the use of a mask, in no case is a greater time than 1½ to 2 minutes
taken. A cycle lamp, small size, takes fully 2 minutes to brush
properly. To spray the same lamp takes 10 seconds. A gas meter, 5 or 10
light size, takes 7 minutes to brush. To spray one takes 1½ minutes,
allowing for the time occupied in fixing and removing the somewhat
elaborate mask employed for covering the indicator glass, badges,
name-plate, etc.
A motor headlight takes nearly 20 minutes to brush properly, whereas to
spray one takes but 1½ minutes at the outside.
To brush a cycle frame takes 10 minutes or so (brushing is still done
for coloured work).
To spray either in japan or colours takes 1 to 1½ minutes, and when it
is considered that a heavier coat can be applied than is the case when
brushing or dipping, it will be admitted that such a result compares
favourably even with dipping.
A 2-seater motor body takes from 25 to 45 minutes to brush properly.
To spray the same body takes only 6 to 8 minutes, and when it is noted
that a fraction of the rubbing down is necessary in the case of the
sprayed result, as against the brushed body, it will be granted that an
enormous gain is to be effected.
Where two or more colours are employed, it is difficult to make
comparisons as readily, but, generally speaking, it is the practice to
employ two instruments per operator where the colours are applied one
immediately after the other, as is the case with many electrical parts,
and in this case a gain of 4 or 6 to 1 is shown over brushed work.
In some cases it has been found an advantage to spray one coat and
to brush the other, especially where some difficulty is experienced
in masking, and, in such cases, the only way to arrive at the most
satisfactory method is by actual test.
In other cases, such as bronzing, with a celluloid medium, it is
absolutely impracticable to apply these bronzes, save with the spray,
unless the old method of varnishing and afterwards dusting on the
bronze is adopted; but, of course, against spraying, such an obsolete
method as the latter has absolutely no chance when speed is considered.
Iron piano frame manufacturers have almost universally adopted the
system of spraying bronze on the frames. Where the old-fashioned method
is still in use, it is usually because the quantity of frames to be
dealt with is hardly large enough to justify the outlay required in the
purchase of a plant.
Another advantage should be mentioned. The special celluloid japans now
largely used for application by means of compressed air obviate the
necessity of a considerable amount of rubbing down and papering, etc.,
which cannot be dispensed with when the liquid is applied by means of a
brush.
For the purpose of comparison, it may be said that these japans may
be sprayed upon such articles as buttons, medallions, electrical and
bedstead fittings, and also on wood brush-handles and furniture, large
and small, with a gain of 5 to 1 over brushing.
OBJECTIONS SOMETIMES URGED AGAINST SPRAYING.
Having explained the advantages of the spraying system, we may now
consider the objections which are occasionally raised against it.
One of the arguments most frequently used is that the effect thereby
obtained must be less durable than is the case of the brushed effect.
Why such a theory should be advanced is a little obscure, for,
theoretically, it is possible to apply, with a properly fitted plant,
a far more even coat than is the case when brushing, and seeing that
such paint is applied with the idea of protecting the article against
rust, or corrosion, or of improving the appearance, the argument is for
rather than against spraying.
However, to come to the practical side, the evidence is all in favour
of spraying, for it has been proved by many exhaustive tests by
independent persons, that, with the same material treated in the same
manner, _i.e_., either air dried or stoved after spraying or brushing,
there is no difference in the durability of the japan or paint, and
that the sprayed result has this advantage, that some little gain in
time is effected in the period occupied in drying or stoving of the
paint.
Another objection is sometimes raised to the application of paint by
means of the spray to rough and uneven surfaces. The argument here is
that it is impossible to make the paint adhere to the surface, save
with a good stiff brush.
Such arguments ignore the pressure at which the paint is applied, which
is usually between 30 and 45 lbs. per square inch, and are mainly based
on unfortunate experiences with early types of whitewashing machines,
which have already been pointed out to be entirely unsuited to painting
problems.
Again here, it may be positively stated that in no case has it been
found that a less durable finish is obtained with the spray. As a fact,
an advantage is to be recorded in that on rough surfaces the gain in
time is enormous, for such surfaces may be spray-painted as rapidly as
the smoothest surface obtainable, whereas the labour of brushing such
rough surfaces is very considerable.
Still another objection which may be mentioned is the slightly mottled
appearance which the sprayed surface sometimes shows when finished--if
such can be objected to seriously.
This objection may be met by the statement that such a mottled effect
is only apparent when the paint has not been specially prepared for
spraying, and is mainly due to the fact that mediums used dry rather
too rapidly and do not allow the paint (which is, of course, applied
in minute spots) to flow out.
The remedy is naturally to re-model the paint should the mottled
appearance be objected to, but most certainly such an effect, if
not pronounced, is not any worse than brush marks, which are always
noticeable unless rubbing down has been done.
In passing, it may be stated that most of the large paint and varnish
manufacturers have experimental plants for producing spraying
materials, and they are naturally well able to answer any queries which
may be likely to arise and which may not be adequately dealt with here.
Many firms who consider the question of adopting a spraying plant seem
to have the idea that while the principle is admirable for many trades,
yet it cannot be successfully employed in their particular case, and
one frequently finds the manufacturer of small goods wondering why his
neighbour opposite, who makes, we will say, motor bodies, does not
install a plant, although, of course, in his own case the question is
impossible. Most probably, at the same time the said neighbour has
precisely similar thoughts, save that he considers the only proper use
for a plant is in small work.
Occasionally one hears the opinion advanced that primers or fillers
cannot be applied with a sprayer, or, if so applied, it is at a risk
that they will either crack or peel off afterwards.
This objection may just as readily be advanced in the case of brushing.
If trouble of this kind occurs, it is not due to the method of
spraying, or brushing, so much as to the way in which the paint has
been used. This statement sounds paradoxical, yet it will be understood
by practical men. For instance, it is obvious that motor bodies, par
exemple, must be dealt with by the expert coach-painter as far as the
methods, number of coats, etc., to be employed are concerned, even
though the work may be carried out by unskilled labour, and naturally
whether brushed or sprayed, motor bodies cannot be well done by novices.
To summarise, it may be confidently stated that whatever difficulties
there may be in the application of paints, enamels, varnishes,
lacquers, etc., by means of brushes in the ordinary way, these are
certainly never accentuated but are in most cases considerably lessened
by the use of an adequate spraying plant.
SPRAYING AND DIPPING COMPARED.
In considering those articles which may be dealt with either by dipping
or spraying, the manufacturer has first to take into account the number
of coats necessary.
If, for instance, to produce the correct finish on a cycle frame, when
dipping, needs three coats, and assuming that the same effect may be
obtained in two when spraying (and this is frequently the case), then
spraying must be considered to rank favourably with dipping, and if
allowance be made for the time taken to drain, and the room occupied
with an elaborate dipping plant, then again spraying scores, for a
cycle frame can be sprayed in 1 to 1½ minutes, or, say, 40 to 50 frames
per hour, which is very little, if any, less than can be comfortably
dealt with by dipping, and, of course, no time is lost in draining.
If, on the other hand, gas is so cheap as to be of little consequence
whether two or three stovings are necessary, and if some of the work
can be done with two coats, then the saving effected when spraying
would not be sufficient to warrant the outlay necessary for a modern
compressed air plant, _unless_ a fair proportion of coloured work, not
easily dipped, was required to be dealt with. In this latter case, a
spraying plant ought to receive very serious consideration.
To take another case. Some gas fires may be dipped, but, owing to their
weight and to the fact that they are seldom entirely free from grit or
dust, they are not a satisfactory type for dipping, and we venture to
state that all the leading gas engineers have dropped the question of
dipping such work in favour of spraying, although in some cases, for
gas cookers, the accessories, such as burners, bars, racks etc., are
dipped.
Such accessories are as readily sprayed, but as only one coat is
applied, naturally no saving is shown over dipping.
Other goods sprayed or dipped are steel implements, hoes, spades,
shovels, axes, picks, etc., and in some cases it is an advantage to
dip where the same colour, black, red, etc., is needed, say, half-way
up the spade or hoe, but otherwise spraying should be adopted; and, of
course, in many cases where two colours are employed it is out of the
question to dip, the only alternative being to spray or brush--spray
for preference on account of the enormous saving of time over brushing.
Where very small articles are dealt with in bulk and can be so handled
that trays may be used, then these are undoubtedly better dipped; but
if various colours are needed it is better to still use the trays, but
to spray them, as the speed is approximately the same as when dipping.
A tray full of camera parts is sprayed almost instantaneously and with
a far better finish than if dipped.
To sum up the various arguments advanced on this subject, we may come
to the following conclusion.
As already intimated, the saving effected by painting by immersion or
by spraying as compared with brushing is very large indeed, provided
that a properly equipped plant is employed, and the paint or enamel is
exactly fitted for the purpose. The question must often arise as to
whether dipping or spraying is the best and cheapest to employ. The
answer is that everything will depend upon the nature of the article
that is to be painted. Speaking generally, painting by immersion
possesses many advantages, because the actual work of applying the
paint is done in a few minutes and every part of the article is covered
with paint at one time. In reaping machinery, iron treads of staircases
and many other parts of the kind in which there are a large number of
small parts or depressions, paint dipping cannot be improved upon; on
the other hand, spraying possesses distinct advantages in dealing with
many classes of goods which are of such a shape or character that if
the paint were applied by dipping some accumulation in parts would
inevitably occur.
A decision on the subject can only be arrived at after a very careful
study in comparison of the two methods. It will frequently be found
that, on the whole, the most economical plan to pursue is to dip the
first and subsequent coats, excepting the last, and to spray this on,
particularly when it consists of varnish or a viscous liquid. It is
in this particular that one can draw a wide distinction between the
two methods. In dipping, the paint must not be very thick, and, as
previously stated, in designing a paint for the purpose the greatest
effort is made to produce one which gives a happy medium between
excessive thickness and the reverse, too thin a coat. Just as soon as
the peculiarities of the article to be painted necessitate the use of
all thick paint or enamel it must be considered that dipping is not
suitable, but that spraying is. The latter may be employed successfully
with any liquid up to the thickness of tar.
[Illustration: Fig. 111.--AIROSTYLE PLANT SHOWING VENTILATOR AND
PORTION OF HOODS FOR 4 OR 6 HOODS PROVIDING FOR SUBSEQUENT ENLARGEMENT.]
[Illustration: Fig. 112.--ELEVATION OF SPRAYING PLANT.]
[Illustration: Fig. 113.--ELEVATION OF SPRAYING PLANT.]
[Illustration: Fig. 114.--PLAN OF FIGS. 111 AND 112.]