Painting by Immersion.
The process of painting various articles by dipping them bodily into specially prepared paint contained in a tank or other receptacle is a very old idea. It has, however, developed considerably in late years, and is now largely employed in many industries, particularly in the finishing of agricultural implements of various kinds, iron work and a hundred and one other articles. It is sometimes used in conjunction with paint spraying; that is to say, the first coat or coats may be put on by immersing the article to be painted in the paint, or varnish, and the final coat of enamel or varnish may be put on by means of a spray. In some cases, the final coat is applied by means of brushes in the ordinary manner.
Perhaps the simplest form of paint dipping is that applied to various articles, such as bolts, rings and small parts, which are placed in a wire basket and plunged into the paint. The basket is then hung up for a quarter of an hour or so to drain, and afterwards in another place, until such time as the paint becomes hard.
Another familiar example of painting by dipping is that of ordinary tapered cans, such as those which are used to contain varnish, etc. In this case, a piece of wood is introduced into the neck, the can is immersed in the paint up to almost the top of the neck, and is then placed upside down to drain and to dry. A paint which dries with a gloss is usually employed for this purpose, and it may be remarked that a good deal of this work is now done by spraying; in fact, a great deal of difference of opinion exists as to the respective merits of the two processes for this particular purpose.
Fig 1.—Cross Section Through Paint Tank.
Going a step farther, we come to the consideration of articles such as iron sashes and casements, parts of staircases, etc., which may be dipped into a tank containing, say, five to ten gallons or so. Such tanks are usually made to[Pg 8]
[Pg 9] slant at the bottom in order to facilitate cleaning when necessary. Adjacent to them is provided a platform, usually lined with sheet iron upon which the paint can drip after the articles are removed from the tank, and a system of overhead rails for moving the articles from one part of the works to another. No agitating apparatus for the paint is required, but after the tank has been used for some time, say, for example, at the week-end, it is necessary to stir it up by means of a pole before the work commences. As a matter of fact, the articles being plunged in the tank, and their withdrawal, in itself stirs the paint sufficiently for the purpose.
In dealing with the larger-sized articles which are to be painted, such as reaping machines, the construction of the tank which may hold several tons of paint is naturally of a more elaborate character. Such a plant will consist, first, of the tank itself; next, the system of overhead railing; third, the apparatus upon which the articles to be painted are hung; and fourth, the hoists for lowering and raising such articles into and from the tank. It will be convenient to consider these parts which go to form a complete installation under their several heads, taking a more elaborate plant by way of example, it being understood that the plant may be simpler in form when the size of the articles to be painted is small, or under other circumstances, such as will be presently suggested.
The Tank.
Stirrers and Blinds for Paint Dipping Tank. Fig 2.—Plan.
Fig. 3.—Longitudinal Vertical Section.
There are two forms of special tanks in general use, one having at the bottom paddles for agitating purposes, and the other worms provided with the same object. Fig. 1. shows a cross section of a tank made on the McLennan patented system. In this two series of paddles revolving in opposite directions are provided, and above them is an appliance which may be likened to a horizontal venetian blind, consisting of iron laths, which are nearly horizontal when closed, forming a platform upon which the paint can settle, and vertical when open. This blind is left open when the tank is in use, and is closed when it is at rest. The illustration gives the dimensions of the parts, although these, of course, will be varied according to circumstances. Above the blind is sometimes a grating, which, together with the blind, form a protection to the agitating gear from articles dropped into the tank by mistake, the two together preventing the heavy, pigment in the paint from clogging the paddles when the[Pg 10]
[Pg 11] agitating gear has been stopped for some time. Figs. 2 and 3 show respectively longitudinal plan and section, from which the construction will be clear.
Messrs. Wilkinson, Heywood & Clark, Ltd., of Poplar, who own the McLennan patents, in a little book, entitled "Painting by Immersion," state that there were certain disadvantages in the design of this tank, inasmuch as should it become necessary to remove or repair the paddles, it would be necessary to empty the tank and bodily remove the parts. They have, therefore, a new design, which is shown in Fig. 4, in which this remedy is overcome. The agitating gear, in this case, is on a sub-frame, separate and independent from the tank itself. The drive is in the tank, and is taken by means of a claw clutch through a right angled bevel to the agitating shaft. This design has proved very satisfactory for small tanks. For very large tanks a gear is recommended in which the agitating gear is enlarged and the shafting is supported or stiffened to prevent whipping. This entails a design of some special bearing, which should be paint proof and oil proof, for the reason that the bearing is immersed in the paint itself. In Fig. 5 is shown drawings of this bearing, on which the firm named hold patents. The perfected plants now consist of a tank with an independent and removable chassis frame, on which are mounted the Venetian blind arrangements, already mentioned, together with the agitating gear, which consists of either one, two, or more parallel shafts, mounted on parallel gear, as shown in Fig. 6.
These shafts are held on the sub-frame with the patent bearings already referred to, and the drive is transmitted through the bevel gear encased in a special gear box, which is equally oil and paint proof and self-lubricating. It will readily be seen that with this plant it is an easy matter to withdraw the driving gear and lift the sub-frame bodily out, without in any way disturbing the paint in the tank. The tank, therefore, can be buried nearly flush in the ground, embedded in concrete, and need never be disturbed. In case of emergency, and when painting is continuous throughout the year, it is recommended that the consumer should keep a complete spare chassis for use when occasion requires.
Fig. 4.—Design for Tank for Painting Steel Sheets.
Fig. 5.—Details of Paint-Proof Bearings and Gear Box.
Fig. 6.—Details of Sub-frame, Shafting and Driving Gear for Tank Designed for Implement Manufacturers.
Whatever the construction of the tank itself may be, and whether or not it is provided with agitating apparatus, it is generally found most convenient to build it with the top nearly level with the floor. It is advisable to provide iron doors or covers to close the tank in when out of use so [Pg 12]
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[Pg 15]as to prevent excessive evaporation and also for use in case of fire. These doors or covers should therefore be actuated by levers or other suitable appliance which may be put in motion at a point some distance from the tank itself, for while the paint is not very liable to catch light, an accident may cause it to do so, particularly when electricity is used in the works, and the closing of properly constructed doors will quickly subdue fire as far as the tank itself is concerned by shutting off access of the air. The usual plan followed is to provide wide strips of thick felt attached to the doors in proper position so that when the doors are closed the contact will be thorough and the tank itself be practically sealed.
When agitators form part of the apparatus it is not necessary to buy the paint ready mixed for use, as it may be introduced into the tank in thin paste form; the necessary thinners such as white spirit can be added and the agitating apparatus slowly revolving will quickly mix the parts together and render the paint suitable for use.
It will be understood that the form of tank above described may be very considerably modified according to circumstances. In very simple paint dipping, such as is required for small articles, casements, etc., the agitating apparatus may be wholly dispensed with, while in other cases, the shutter-like arrangement above mentioned may be omitted and the agitator may consist of a worm with a screen above to protect the parts from pieces which may accidentally fall in.
Fig. 7.—General Lay-out of a Three-tank Plant Designed for the Use of an Agricultural Implement Maker, Using Three Colours in this Instance, Red, Green and Blue.
The Rails and Hanging Apparatus.
Fig. 8.—Joist and Wheels Supporting Hanger.
These two details may be conveniently considered together, and it should be stated at once here again a great deal of difference of opinion exists as to what system is the best. It should, however, be clearly understood that upon the system of railing a great deal of the success depends. The object, of course, to be attained is that of handling a large number of parts with as little labour as possible. With this object, a complete system of overhead rails, commencing with the finishing shops, leading to the paint tank, and thence to storage or delivery departments, should be carefully thought out, always remembering that the growth of a business may necessitate the erection of additional buildings from time to time and provision made accordingly. One of the simplest and best forms of rails is an H section joist with two wheels on either side as shown in Fig. 8. In other cases an L rolled joist is used, the railing part being on the vertical flange while the horizontal flange is connected with rods fixed to the upper part of the building. One method which is used quite successfully is to employ hooks as shown in Fig. 9. These run over rails which are lubricated with vaseline, and in this case the rails are usually slightly inclined. In very large works, however, where a considerable length of rail is employed this inclination might be inconvenient, as the rails would go too near the ground. In all cases horizontal rails are found in practice to be the most convenient, but an installation requires careful consideration from all points of view and no general rules can be laid down.
Hanging after Dipping or Spraying.
A practical point of more importance than might be supposed, at first sight, is to avoid hanging articles very close together after they have been painted or enamelled, either by dipping or spraying. The reason is that if they are hung quite close together the turpentine or volatile thinner used in the paint may affect part of the adjacent surface, and cause a diminished gloss.
Fig. 9.—Hook for Hanging.
This point was brought to light in the case of one manufacturer who was painting varnish cans by dipping. They were hung up to dry quite close together—in fact, almost touching. When dry, it was noticed that, instead of being nice and glossy all over, one side was somewhat dull. The explanation appeared to be that the turpentine fumes affected the paint in drying, and this was proved to be true, as when the cans were placed farther apart the trouble ceased.
Hoists.
The hoists used will depend upon the size and weight of the objects that are to be dipped. In some cases they may be dispensed with altogether and the dipping can be done by hand.
Fig. 15.—A Typical Hoist for Use in Painting by Immersion.
In cases where the space is confined, it is sometimes desirable to use hoists in the form of wall crabs, and a few of these made by the London Hoist Machinery Co., Ltd., 103, Worship Street, E.C., are illustrated in Figs. 10 to 14. The hoist shown in Fig. 10 will lift 10 cwt. with slow speed on the left-hand side, and 3 cwt. on the right-hand side with[Pg 19]
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[Pg 21] a quick speed of 13 feet per minute. This form can be made lower by brake if desired. The little crab shown in Fig. 11 deals with weights of 1 cwt. at a speed of 60 feet per minute, while that shown in Fig. 12 is also a quick gear, lifting 3 cwt. at 13 feet per minute. The other illustrations are self-explanatory.
Fig. 15 shows a typical hoist which may be regarded as an ideal installation for the work, and the idea can be adapted to either large or small works as required. The installation consists of an overhead track with a movable portion over the dipping tank. The goods to be dipped are hung on the trolleys and run one by one on to the movable portion of the track over the tank. There is a stop on this portion to prevent the trolley running right over the tank, and the movable portion with the article to be dipped is lowered into the tank, and afterwards raised by means of a hoist. The hoist in this case was driven by pneumatic power. It could, however, in very small installations be a hand hoist worked by the rotary movement of a handle, or in other large installations a quick moving electric hoist.
After the article is dipped and the movable portion raised by the hoist to its correct position in the track of the runway, the trolley is run off the movable portion on to the track beyond the tank, and left there to dry.
These overhead runways can be made to suit loads from 2½ cwts. up to 10 tons, so that the system covers all classes of work, since it is seldom that it would be necessary to dip articles weighing more than 10 tons.
Fig. 16 shows a Morris standard electric trolley hoist suitable for lifting up to, say, 5 or 6 tons. Both of the foregoing hoists are made by Messrs. Herbert Morris, Ltd., of Loughborough, to whom the author is indebted for these illustrations.
In Fig. 17 is shown another type of trolley hoist suitable for lifting heavy weights.
The number of Coats of Paint to be Given.
An important consideration both in paint immersion and paint spraying is the number of coats of paint which should be given in order to produce the required appearance and ensure durability. This must necessarily depend upon the use to which the article to be painted is to be put, and it will also depend upon the appearance. In paint spraying it is clear that any thickness of paint required could be obtained by continuing the operation of spraying, but it has been found in practice that it is not desirable to continue the application of the paint beyond the point when every part of the surface is completely covered. It is a well-known fact that three thin coats of paint applied separately will prove more durable than two thick coats, even if the quantity of paint used in the two cases is identical. The same is true to a great extent when the paint is sprayed on. When it is applied by dipping another important element enters into consideration, and that is, thin coats are a practical necessity, because if an attempt was made to make the coat thick it would inevitably result in unsightly runs, fat edges, etc., which would wholly spoil the work. Where the object is only that of a temporary protection, as in the case of iron castings, a single coat will usually suffice, because the castings when fixed in position will be painted with the building in the ordinary way.
Sometimes a priming or first coat of paint is given by dipping, and the following coats are applied by brushes or by spraying. There appears to be some idea that when paint is applied by brushes it is forced into the pores of the wood and therefore holds better; as a matter of fact, in a well-designed paint the thinners will penetrate the wood and no particular force is necessary; but if it should be, it will be certainly supplied by the spraying process.
Advantages of the Dipping Process.
The advantages of applying paint, japan, enamel, or varnish by the process of immersion are obvious. The saving of time is the chief advantage, and this will be in most cases very considerable. For instance, a complete wagon can be painted by dipping in a very few minutes, while large and intricate agricultural machines may be painted in the same way in a fifth part of the time it would take to spray them, and probably a twentieth part of the time it would take to do the work by hand.
There is another advantage which should be mentioned when paint dipping is compared with spraying. In the latter, an outfit consisting of sprayer, air compressor, exhaust, cabinet, etc., are all necessary. In dipping, however, the apparatus may in many cases be of the simplest character. For example, in painting bedsteads all that is necessary is a narrow but deep tank containing the paint, into which the bedstead may be plunged by hand and then hung up to drip over a metal-lined floor.
Fig. 16.—A Morris Standard Electric Trolley Hoist.
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In Birmingham, hundreds of iron bedsteads are dipped in this way every day. The head or foot of the bedstead is taken in the two hands of the operator, dipped into a tank of black japan, and immediately hung up on a travelling chair, which slowly carries it across the room to the oven, the superfluous paint dripping off during the process.
In another chapter will be found the advantages which the process of spraying has over dipping. A decision as to which is the best system to adopt for any particular purpose can only be determined after due consideration has been given to all the circumstances bearing upon the work to be done.
It may be pointed out in this connection that the process of painting called "flowing on," which is fully described on another page, is in effect only a modification of dipping. As a motor body, for instance, cannot be plunged into a paint tank because the inside is not to be painted, the paint is literally poured over the surface, and the effect is exactly the same.
Protecting Parts Not to be Painted.
It frequently happens in paint dipping that certain parts of a machine or other article that is desired to be coated with paint is to be left unpainted, and the problem is how to effect this most economically. The method usually employed is to cover the parts, such as name plates, bright portions of a machine, etc. with vaseline. When the article is dipped the paint covers this as well as the other parts, and after the paint is dry the vaseline and the paint on it can be easily wiped off and the surface beneath it will be found to be quite clean.
Preparing Woodwork Before Painting.
Previous to the priming coat of paint being applied to any article made of pine or other wood containing knots it is necessary to protect such knots by applying one or two coats of a liquid known in the trade as "knotting." If this were not done the rosin which exudes more or less from the knots would penetrate the paint, discolour it and give a very unsightly appearance.
The best knotting consists of shellac dissolved in alcohol, usually in the form of methylated spirits; in other words, it is shellac spirit varnish. Many inferior grades, however, are sold in which the shellac is adulterated with rosin or other substances, while the alcohol is sometimes replaced wholly or in part with naphtha. These inferior qualities of knotting should never be used, as they are very likely indeed to cause trouble and spoil the whole job.
Although the parts of machines such as agricultural implements are usually made of selected and well-seasoned timber, it sometimes happens that portions of the work are found to be more or less sappy. This should also be coated with knotting, as otherwise they will absorb the paint to too great an extent and the priming coat will not be uniform.
In passing, it may be observed that a coat of shellac varnish—a term, as already explained, which is synonymous with knotting—is very useful as an undercoat whenever there is an excess of rosin as in pitch pine. It is also used to stop suction on plaster ornaments which are finished in imitation bronze, and which may successfully be used over tar or tar spots which it is desired to paint.
It is very important that knotting be applied in quite thin coats, and it is for this reason that usually two coats are given. If it is too thick it fails to dry properly. Thick knotting may be used if it be well brushed out, but this is almost impossible in applying it to the knots of ordinary woodwork which are merely "dabbed" with the end of the brush; moreover, such work is usually done by piece work, hence two coats are much safer to use.
Filler For Iron.
It is sometimes necessary before dipping or spraying cast iron work to fill up sand flaws and inequalities. This may be effectually done by using a filler prepared as follows: Mix together equal parts of genuine red lead and gilders' whiting and add two parts of boiled linseed oil and one part of goldsize. It will be found most effective to mix the two liquids last mentioned before adding them to the dry lead and whiting. Thoroughly mix all together, or, preferably, grind through a mill until the putty-like consistency is obtained. This stopping will dry very hard. It is important to remember that this filler must be mixed in small quantities as required as it rapidly becomes hard, so that if a considerable quantity were made at one time any unused would become spoiled.
Fig. 17.—Electric Hoist Suitable for Heavy Goods.
Another recipe of an entirely different character for a cement or putty for the same purpose is made by mixing one parts of flowers of sulphur, two parts of sal-ammoniac and 80 parts of iron filings and making them into a paste with [Pg 27]
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[Pg 29]water. By increasing the proportion of sal-ammoniac the setting is also increased.
Stopping Holes in Woodwork.
However carefully a piece of woodwork may be prepared, there are usually more or less depressions or holes such as those which occur over countersunk nails, and in other places which require to be filled up. The usual stopping for this purpose employed by house painters is a mixture of equal parts of dry whiting and dry white lead, made in the form of putty, with raw linseed oil to which, say, 10 per cent. of boiled oil has been added. A cheaper and quite as effective a stopping, however, can be made by using a material known as "Alabastine," which possesses the advantage of being easily cut down. This material may be also used when mixed to a thinner consistency for priming, but it is not suitable for being applied by dipping, although it may be sprayed. The very best stopping is one such as is used by carriage painters, and is strongly recommended for high-class work. It is supplied both in powder and paste form by all first-class varnish manufacturers, and although the cost is a little higher than that of stopping putty made of whiting and linseed oil, as described, it is well worth the difference.
An excellent filling composition for coach and motor body builders, railway and tramway carriage manufacturers, etc., is that made by the well-known varnish house of Messrs. Wm. Harland & Son, Merton, S.W.
It is especially suitable, obtaining a perfectly level and unyielding groundwork where a fine finish is to be given, either in paint, varnish or enamel. It is made in two colours, grey for dark work and cream for light colours or white, and costs sixpence a pound, or less when bought in quantities.
It effects a great saving of time and labour, being already finely ground into a stiff paste, which only needs the addition of specially prepared thinners to bring it to a proper consistency for use with the brush.
It is most successful and expeditious as well as the most economical process for giving a smooth surface to rough castings—a condition which is, of course, indispensable when these have to be painted and varnished.
For ordinary surfaces of woodwork of a slightly rough or uneven character, two or three coats of the filling composition will suffice to produce, when carefully rubbed down, a perfectly smooth surface of a hard metallic nature, combined with great tenacity and durability.
"Philorite" is another speciality which may be mentioned in this connection. It is manufactured by Messrs. Wilkinson, Heywood and Clark, Ltd., and is a filler made in white and seven colours. This is a great advantage when the finish is to be coloured, as it may save a coat of paint. It can be rubbed smooth without difficulty, and will serve as a perfect ground for enamels and japans, as well as for paint. For use it is thinned out with American turpentine to the required consistency, say, that of a ready mixed paint. It is brushed on with a stiff bristle brush, and two coats may be safely applied on one day. The last coat can be rubbed down smooth with pumice and water or sandpaper. It may also be used as a knifing paste for defective surfaces.
Rubbing Down.
In many cases the coats of paint, japan or enamel have to be rubbed down between coats in order to produce a perfectly level surface. This is usually done by means of finely powdered pumice stone and water, and it is important to remember that the operation must be done with care so that the rubbing is quite uniform. If a slight gloss is given to the paint the portions which are rubbed down can readily be discerned, as they will have a gloss, and in this way uniformity can be ensured. It is of the utmost importance that all of the powder be washed away before the next coat is applied; if any is left it will mar the job. It may be noted that the work of rubbing down may be done more expeditiously by means of the finest grade of steel wool. On the Continent this is now used exclusively, and it is rapidly gaining favour in Great Britain.
Fig. 18.—Sprayed Show Card.
Fig. 19.—Dipping and Stoving Bedsteads.
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