Stencils and Masks—The Requirements of Different Trades in Spraying.

Just as in applying paint by immersion some parts which are not required to be painted can be protected, as already explained, so in spraying.

Certain parts are to be left unpainted, such as the plated fronts and glass lenses of cycle lamps. This is effected by the adoption of some system of masking. Such masks are, however, very simple and are sprung on with a simple wire spring or clip, and so held in place while spraying.

A mask has been designed and patented purposely for use when spraying. This is described on page 143. This mask has embodied in it a plunger on the end of which is mounted a rubber sucker, and as the plunger is controlled by a spring, all that is necessary to do is to depress the plunger down on to the lamp, box, or whatever article is to be painted; and as the sucker of the plunger comes into contact with the article first, it grips the surface and the spring behind the plunger forces the mask, which may be any shape, but is preferably of box form, down upon the article. This mask is very practical, but its use is limited, to some extent, to forms of lamps which do not need any stencil or mask. Such a step, needless to say, has only been taken because the system of compressed air japanning is the one ideal method—and one has only to refer to manufacturers who have used it for confirmation of this.

In addition to simple masks, there are many more complicated forms, such as those in use in gas meter manufactories, where there are no fewer than half a dozen points to be masked, and in hardly any case do more than two points remain the same over numbers of meters to be dealt with.

However, this has been overcome by arranging the masks on a spring foundation, and making them capable of sliding adjustment in two or more directions, and the time taken to fix these masks is in no case more than 30 seconds. Seeing that the time taken to brush a meter (even a small one) is not more than seven minutes and the time taken to spray one, with a superior result, is not more than one minute, there is a very considerable saving even after allowing time for fixing the mask.

Other stencils are employed for lettering on plates, despatch boxes, and for ornamental work on cash boxes, etc., and need no further description here other than stating that these must be of something stronger than tinfoil, and not too strong to prevent them being pressed firmly down on to the article to be dealt with.

In other cases, stencils are spun or pressed out to exactly fit the article, as in the case of brass lacquered goods, rubber balls, enamelled ironware, etc., and in some cases, where a number of stencils are employed, for one pattern, these are arranged round a common centre, and hinged so that, once the article is in position, stencil after stencil may be readily and accurately brought into position over the work.

For decorative work, very elaborate stencils are employed, with many plates for each design; but such stencils hardly come within the scope of the present work, although they are employed for wallpaper work, and in the textile industries for high-class effects.

The Hart Patent Mask.

A very ingeniously constructed, although simple, mask which is suitable for many purposes, but particularly for motor and carriage lamps, has been patented by Mr. Walter William Hart, of the firm of Matthew J. Hart & Sons, Great Barr Street, Birmingham, and others. By this invention means are provided for applying the mask to a plain surface so that it may be immovably held without causing injury to the surface. The mask, too, has the advantage that it can be placed in position with extreme rapidity. By this invention the mask is fastened in position by a suction pad, preferably made of indiarubber, which, when pressed on to the surface to be covered, adheres thereto by the pressure of the atmosphere. In Fig. 93 is shown an inner face view and a sectional elevation of the device. The mask (a) is adapted to have an axial movement in relation to the suction pad (b) so that when such pad has been fixed in position the mask may be pressed close upon the surface it is required to protect. This suction pad (b) is of a conical formation, and is secured to the end of the plunger (c) which passes rearwardly through a hole (d) in the back of the mask, and through a tubular extension (e) passing rearwardly from the back of the mask. The rear end of the plunger has a hole therein, which hole is screw threaded, and to such end a cap (f) is secured. A stud (f) on the interior of the flat end of the cap screwing into the said tapped hole, and the cylindrical portion (f2) of the cap slidingly fitting over the tubular extension (e) on the mask. A spiral spring (g) surrounds the plunger, one end bearing on a shoulder (h) at the rear of the mask and the other end bearing against the cap (f). To fasten the mask, the same is placed in position over the part to be covered and the cap (f) is pressed down, thus pressing the suction pad (b) upon the surface of the article, to which it adheres. A mask constructed according to this invention is capable of a great variety of applications, one of which is in connection with lamps. That shown in the illustration is intended for this purpose.

It will be clear that the shape, however, can be varied according to the article to be painted by spraying; for example, it may be used with advantage for decorative purposes.

MAKING STENCILS.

When it is decided to spray through stencils, it is usual to employ thin copper plates for the purpose, although in wallpaper decoration very heavy copper stencils are used, because, these being held horizontally, the weight increases the stiffness of them, while it prevents them shifting while in use. In ordinary work, however, where a stencil must be used vertically, the lighter stencil will be most useful. If paper is used, the best material is Japanese vellum, which is manufactured from a special fibre and possesses the advantage of being easy to cut and makes remarkably durable stencils. It is imported from Japan by Jas. Spicer & Sons, Ltd., 15, Upper Thames Street, London, E.C. Manilla paper may be had from the same firm, and answers also very well. For some styles of work, lead foil gives good results. Messrs. Locke, Lancaster, and W. W. and R. Johnstone & Co., Ltd., 94, Gracechurch Street, E.C., supply a material which they call "Four-ounce stencil metal," which weighs 4 ounces to the square foot, is made in sheets 6ft. by 2ft. 6in., and in smaller sizes, and costs 2s. per lb. It can be easily cut with a stencil knife, and is most suitable for work on horizontal surfaces. Another paper which may be used is ordinary Whatman's paper. The usual method of cutting is to use a very sharp knife, such as a shoemaker uses, and to cut on a sheet of glass. If any variety of paper is employed, it will be necessary to apply two coats of shellac varnish, ordinarily called "Patent Knotting." This protects the surface of the paper from being worn away by the action of the brush. Some stencil cutters prefer boiled oil for the purpose.

Zinc stencils are sometimes employed. These may be made as follows: The thinnest sheets of zinc are employed for the purpose, and upon this is painted the letters or design to form the stencil. The whole of the zinc which is not to be cut out, or rather, by this process, eaten out by acid, must be protected by means of a varnish made as follows: Take one pint of best asphaltum varnish, two ounces of beeswax and half-an-ounce of rosin and four ounces of Venice turpentine. Melt the beeswax and rosin in the Venice turpentine, and while warm add the asphaltum varnish, mixing thoroughly together. Paint this on the back of the zinc plate and on all parts that are to be protected. Form a dam or little wall made of 6 parts of beeswax and one part of tallow melted together and allow to cool. This is intended to confine the acid to the parts required to be eaten out. Nitric acid is used for the purpose mixed with three parts of water. This is poured on the space inside the dam and allowed to remain on for from 24 to 48 hours, when it will be found to have eaten away the zinc on the parts unprotected. Further information concerning stencils will be found in the book entitled "Stencils and Stencilling," by A. L. Duthie, published by the Trade Papers Publishing Co., Ltd., 365, Birkbeck Bank Chambers, High Holborn, London, W.C., price 3s. 3d.

REQUIREMENTS OF SPECIAL TRADES.

The following hints are given of the requirements of a few special trades:—

Piece Work.

In very many cases where articles are painted by means of compressed air it will be found quite practicable to have the work done by piece work. The exact time taken to spray any particular article can, after a very little practice, be determined quite accurately. In fixing the price to be paid for the work an allowance must, of course, be made for the time taken in bringing the piece to the spraying cabinet and placing it ready to be taken away by the boys or labourers. It is in this detail that speed may be increased so materially. The actual time of spraying to a great extent regulates itself, but if care is not taken considerable waste of time will be occasioned in getting articles ready and taking them away, and a satisfactory arrangement of piece work to both employer and employee will remove this waste of time and will prove more beneficial than day work.

Bookbinding.

The principal use of spraying in this work is to put on the speckled edges of books after they are sewed and before binding. This is usually done by splashing colour from a stiff-haired brush over a comb, but the method is a crude one at best. In using a spray for the purpose one of the simpler forms will do all that is required. The pressure must be lowered and paint removed some distance from the books until the requisite size of the spots or dots of colour is obtained. With a little practice this can be regulated to a nicety.

Carriages.

The remarks which will be found elsewhere, under the head of "Tramcars," apply to some extent to carriages also, and although the superfine finish required for a high-class carriage could hardly be produced excepting by the old-fashioned means, yet the processes of flowing-on and spraying could, without doubt, often be employed for the cheap and moderately cheap style of vehicles of various kinds which are now produced on so extensive a scale.

Cycle Parts.

Many parts of cycles are now painted by means of spraying, although some firms prefer dipping. Whichever process is used it is necessary, as a rule, to give at least two or sometimes three or even more coats, and to get a fine finish to rub down between these coats with powdered pumice and water, and, for the last coat underneath the finish, rottenstone and oil. The parts must always be stoved, as air drying enamels are not, as a rule, sufficiently hard to permit of their resisting the hard wear to which cycles are subjected. It need hardly be said that the method of handling various parts will to a great extent determine the economy effected.

The wearing parts of cycles are sometimes finished in the following manner, which is the method used by the Birmingham Small Arms Co. Although somewhat expensive it is certainly worthy of all praise, because it gives so durable a finish. The iron or steel parts are first thoroughly washed in American turpentine and are then stoved, the result being a perfectly clean surface. A coat of Calcutta linseed oil is then given by means of brushes and the parts are then stoved at 250° F., after which they receive a coat of what is called "Black Rubber Solution," an elastic enamel which dries with a semi-gloss. Then two separate coats of khaki colour japan are given and each are stoved at 280° to 300° F. A little rubbing down between the coats is sometimes necessary, and this is done with glass paper. The parts thus finished are very suitable for work which is to be subjected to very hard wear. At this writing the cycles are all intended for Government, hence the care taken in the finish.

Electrical Work.

Painting by compressed air is done in the case of many parts of electrical apparatus, the process being similar to that previously described. Thus the metal work of fuse boxes may be finished in this way with black japan, and one coat stoved at 250° F. or thereabouts will usually be found to suffice.

Among the large number of electric works where the process is employed may be mentioned those of W. T. Henley's Telegraph Works, Ltd., at Gravesend. The Airostyle is used in this case, and the work is done approximately from twice to three times as rapidly as it was formerly done with the brush.

Fancy Baskets.

Fancy baskets which are made of cane, willow, special straw and other materials can be sprayed with admirable effect in many cases where brushing would be impossible. Indeed, the application of a spray in this work may be considered to have created almost a new industry in a wide range of fancy goods, which may be sprayed with gold paint, or a great variety of coloured enamels may also be employed. In this case celluloid varnishes are also employed with the addition of coal tar dyes for the production of colours such as mauve, blue, moss green, rose pink, amber, orange, fire red, navy blue, etc. The work of spraying in this case is usually done over a bin such as is shown below, a simple wire device being used at the top to support the basket while it is being sprayed, although even this is often dispensed with as the article may be held by the handle, and being very light this is not found to be irksome.

Gas Meters.

At the present time less than half a dozen gas meter manufacturers or repairers have a paint spraying plant in use, and these are described elsewhere. The saving, however, by spraying the paint is in this class of work very considerable, amounting to about 1 to 5; that is to say, five times as much work can be turned out with a spraying machine as by hand. It is generally recognised that the quality of the painting is much improved, it being harder and better in appearance and generally more satisfactory. Turn-tables of a simple character are employed, and 24 hours are allowed for the drying of the work. For protecting the indicators, the name of owner or manufacturer and brass plates of similar character, various types of shields or masks are employed, consisting of a plate of requisite shape soldered on to a piece of stiff wire, with turned ends, which embrace the meter and hold it in position. There appears to be no reason why the larger size gas meters should not be painted by spraying, provided that a suitable plant, which would be of a simple character, were provided. There is but little doubt that in a little time every manufacturer of gas meters will find it necessary to employ a spraying plant.

Gas Stoves and Ranges.

In this class of work spraying is used almost exclusively; as only one side of the iron is to be painted the dipping process is obviously unsuitable. The work is done at a pressure of about 45 lbs., but sometimes a little less is sufficient. The usual exhaust is provided and turn-tables are employed having bearing wheels on the outer edge of a circular plate. Those used for the larger ranges are usually level with the floor. In addition to black, all kinds of colours may be used. This work is mostly done piece work. The smaller parts, such as rings, cookers, etc., are also sprayed. The stoving is done at 350° F., but when two coats are given the first is subjected to 450° F.

The interior of the ovens are coated for a finish of oxide of tin, which when stoved at a high temperature assures a great increase of "body" or opacity. Sometimes the finish is left solid, but in most cases the coat is stippled.

The usual overhead runways consisting of hanger on four wheels on V-shaped girder are employed.

Picture Frames, Picture Moulding, Etc.

One of the industries in which paint spraying is strikingly successful is that of preparing picture frames with a "gold" finish. If properly done the result is even better than if gold leaf is used, as the spray covers completely the most elaborate and intricate ornament. Picture frames or mouldings of various shapes and sizes are obtained ready for receiving the various coats necessary to give a gold finish. Formerly the whole of the operations were done by means of brushes, but this method has been wholly superseded by spraying, which approximately does the work five times as quickly and gives a far better result.

Three separate processes are employed; the first is the application of a coat of enamel, the second a coat of bronze or "gold paint," and the third the application of a transparent lacquer. The last protects the bronze from discoloration, and the manufacturers guarantee that such mouldings will last at least 5 years without tarnishing.

The whole of the work is air dried, but to facilitate the operation the workroom is kept at a temperature of about 75°F and the drying ovens up to, say, 80° or even 85°F. The first coat of enamel is sprayed on, and in about two hours is felted down with a pad similar to that used by french polishers. This is dipped in a solvent which removes all inequalities. The actual spraying is done on a long open trough shaped bin some 3 ft. 6 in. from the floor having exhausts at the bottom and thin laths placed across it at intervals of about 18 inches. The long stretches of moulding are placed on these laths lengthwise, the laths holding them in position. The spraying is done directly on them and they are turned from side to side, and when the spraying is completed each length is lifted bodily by two boys, one at each end, on to racks in the drying room which is close by and which has a large number of open iron shelves running from end to end. These shelves are quite close together, so that a large number of lengths of moulding may be dealt with at one time. Heat at about 85°F is admitted at the bottom, and the drying apartment is open from end to end in front so that the mouldings may be readily introduced. This heat also serves to warm the room.

The enamel being dry the length is removed and gone over with a pad as described and the gold paint is then sprayed on over the whole of the work, giving a beautifully fine and compact gold finish. The application of the transparent lacquer completes the operation. The materials used in this work are special spirit varnishes made on a base of celluloid. The exact composition varies with different manufacturers, but the following recipes are based on "The Manufacture of Spirit Varnishes," by Livache and McIntosh (Scott, Greenwood and Son), and will be useful for reference. The great advantage of using celluloid is that a coloured transparent varnish is obtained.

Recipe for Celluloid Varnish.

Picture frames are also finished in black having a semi-gloss finish,which gives an excellent effect and is most durable.

It is very important in this class of work that ample ventilation be given to the apartment in which the work is done in such a way as to avoid draught. Heated air, of course, takes up moisture much more readily than cold air, and unless means are provided for adequate ventilation the air will become moisture charged, and the work will thereby be adversely affected. This is a point which is frequently overlooked.

Spraying Ships' Hulls.

It is the opinion of the author that the painting of ships' hulls may be done by means of spraying with a great saving of time. Although there are no available statistics, it is probable that many thousands of acres of surface on ships' bottoms are re-painted every year. A careful enquiry has shown that some experiments have been made in this direction, but that they have always been of a somewhat perfunctory character. The very large surface gives an opportunity for the use of the spray which is almost unequalled in its possibilities. There are, however, certain difficulties in connection with this work, which have doubtless had much to do with the fact that it has not, thus far, been adopted to any considerable extent.

The first of these is that the composition applied is not ordinary paint, but one which is mixed with arsenic, mercury and other poisonous compounds, which are added in order to destroy animal and vegetable life and prevent the adhesion of barnacles and other marine growth. So necessary is this that compositions are made suitable for different waters through which the vessel must pass, and it is well known that those vessels which sail by regular routes need much less frequent painting than a tramp steamer which may sail in any direction and through any sea. The reason is, that in the first case the composition is specially made to withstand the growth of life in well-defined waters, while in the case of tramp steamers it must be more or less of a general nature.

The point, however, which has now to be dealt with, is that the addition of arsenic, etc., added to the paint, causes it sometimes to settle out, but it is suggested that this could be easily overcome by an agitating apparatus in the paint pot. As explained elsewhere, such an apparatus is frequently used in ordinary painting, when the composition of the paint demands it.

The second and more serious objection to spraying ships' hulls is that the work must be done in the open, and that the wind will carry away a portion of the paint and prevent it reaching the surface required. It is suggested, however, this might perhaps be overcome by adopting a form of enclosed cabinet, which would screen the operator from the wind. This could be rendered portable by being mounted on a platform with wheels, and a system of elevators, and be adopted with a little ingenuity to reach the higher parts of the vessel. In any case, the subject is one which is well worth the attention of those interested in spraying, owing to the immense field it opens up.

In this connection attention may be directed to the illustrations which appear on pages 252 to 254 of the apparatus used for spraying by the Pennsylvania Railroad for painting their freight cars. This apparatus may be adapted, with suitable modifications, for ship painting also.

Slate Enamelling.

The usual procedure followed in connection with enamelling on slate for mantel-pieces, stall-board, signs and other work of the kind, is to get the slate slabs quite smooth and level by means of a slate planing machine. A coat of "black varnish" or enamel is then sprayed on, and is stoved at 160° to 170° F. Careful rubbing down with powdered pumice and water is next done, when the work receives a second coat of enamel. It is again stoved and then rubbed down by hand to a finish with rottenstone and oil. Sometimes, the slate receives three coats, the finish being left with a high gloss, but more frequently the semi-gloss obtained by rubbing with rottenstone is preferred. It is unsafe to stove slate at higher than 260° F., as it is likely to break when cooling.

Formerly, a great deal of this class of work was done in imitation of various marbles, mostly worked on a black ground, but "marbling" is now done but rarely. The method is to employ a shallow tank of water upon which are placed colours in imitation of the veins of the marble to be imitated. This colour is of a special kind, made to float, and is manipulated on the surface. The slab of slate, which has received one coat, is then dipped gently in the water, when the colour adheres to it. A coat of stoving varnish completes the operation.

Many slate enamellers still employ the old method of applying the enamel by means of brushes, but it is only a question of time before compressed air will be used almost exclusively, for the reason so frequently urged in this work, viz., that the spray eliminates brush marks and, therefore, greatly reduces the work of rubbing down.

Spraying Colour Specimens.

Specimens of various colours in distemper, oil, varnish and enamel are sent out in very large quantities by the various manufacturers of those goods, and the preparation of these specimens may be very considerably facilitated by using a suitable spraying machine. When distemper is used fairly large sheets of paper are usually employed, and these are cut up by means of a guillotine machine when dry. Ordinary oil paint and varnish paints may in like manner be sprayed, but when an enamel finish is desired it is usual to apply the paint on the back of a sheet of thin white celluloid, which gives the appearance which would be obtained by the application of varnish. In spraying celluloid sheets it will be found that after a little practice the paint may be laid on much more uniformly than is possible by means of a brush. This can easily be demonstrated by holding up the painted sheet to the light, when it will be found that the paint applied by a brush shows more inequalities or streaks than that when sprayed.

Stencilling may be done very well on either paper or celluloid sheets; in the case of these specimens, for example, supposing it was desired that each specimen of colour when cut up should bear a number, it would not be difficult to cut a stencil in zinc or leadfoil, repeating the number as many times as there are specimens to be cut out of each sheet. The tinfoil would then be fixed in position and the spraying done over it, which would mark each piece with a number. Then the coat of colour can be sprayed all over the surface. The plan above would be followed in the case of celluloid, but when ordinary paper is used the process would, of course, have to be reversed and the number put on last.

In some cases where a very great brilliancy of colour is required, this is done by the process of glazing, which is described elsewhere in this book; for example, a comparatively dull red could be very much brightened by spraying a coat of madder or crimson lake. Here, again, if celluloid is used the glazing colour will go on first and the ground colour afterwards, but in ordinary circumstances the ground work would, of course, be first applied.

Probably the most complete plant for preparing colour specimens is in use by Mander Brothers, the well-known paint, varnish and colour manufacturers, of Wolverhampton. The spraying of colours is mostly done on celluloid sheets, and four cabinets of the Airostyle type are in more or less constant use. These cabinets have glass sides and top hung on hinges so that they may be opened when desired. The pressure of air is usually from 30 to 35 lbs., with an additional 5 lbs. on the paint pot. One coat is sufficient, because, of course, it can be made as thick as may be necessary according to the particular specimens being prepared. The saving of time is estimated at, at least, 33 per cent.

In dealing with celluloid sheets it is obviously of importance to adopt some safe means of storing them while drying. Formerly Messrs. Mander Brothers had a long series of double clips by which the sheets were suspended while drying, but this method has been abandoned owing to the danger involved of fire. Now suitable tin-lined cabinets are provided having from top to bottom a series of cleats which support open wire trays. Each sheet of celluloid as it is painted is placed on a wire tray which is slipped in the cabinet, where it dries in 12 hours.

Tramcars.

As far as the author can gather information, the processes described in this book are not used in Great Britain to any extent in the finish of tramcars, but are gradually making headway in America, particularly for the final coats. There appears to be no reason, however, why "flowing-on," as described in Chapter XIII., should not be used to a much greater extent than is at present the case. The earlier coats, being very stiff, might be applied by means of brushes,[Pg 155]
[Pg 156]
[Pg 157] and these coats could be followed by spraying, and finally the finishing coats could be flowed on.

Formerly, this work was done almost in the same way as that by which the fine finish of a carriage body is produced, viz., the building up of a perfectly smooth surface by a large number of coats, rubbing each one down carefully and finishing with two or more coats of varnish, these being also rubbed down excepting the last.

In modern practice these methods are greatly simplified, and at least several coats, which were formerly thought to be necessary, are dispensed with. The finish is often obtained by applying a coat of a suitable enamel, rubbing this down, and then giving a final finish of a good flowing varnish.

It is suggested that spraying, or flowing-on, could be used for some of this work with very great advantage in the saving of time.

In dealing with many paints which are to be stoved, a well-thought-out system should be in use of conveying the articles from the finishing rooms to the painting apparatus and thence to the ovens. If more than one coat is given it will be necessary to arrange also for conveyance after the parts are baked back to the painting plant for the next coat.

As a rule, a system of overhead runways, such as is described for use in paint dipping, will be found the most economical method to adopt.

It will be observed that in the list of trades and articles which may be successfully painted by mechanical means, given on pages 3 to 6, there are many which are omitted from the detailed descriptions. The reason for this is that the main principles are in all cases practically the same and the details must necessarily vary, not only with the goods being dealt with, but according to the extent of the operations and the accommodation of the factory available.

Thornley & Knight, Ltd., of Birmingham, specialise in this business, and have sold, during the last few years, many thousands of gallons of their air-drying coloured japans, for use on car bodies. Many of the largest motor car manufacturers have adopted the process.