THE PAINTING OF BUS BODIES.
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A Consideration of Further Methods of Preserving and Decorating Bus Bodies, Including a Complete Explanation of the Cellulose System.
TN the first portion of this article, published in last -I-week's issue of The Commercial Motor, the writer dealt fully with the paint and varnish system of preserving and decorating bus bodies, particular attention being paid to the preparation of the surfaces to be treated, the application of the priming coat, the fillingup which enables a smooth and hard foundation to be built up, stopping up, the application of the coats of colour and of varnish. In the notes which follow we deal with further details, and then with the cellulose system which has lately come into vogue.
Enamelling is a process often adopted because it saves time, the only disadvantage being that the final process utilizes a medium of somewhat thick consistency which requires skill and experience in order to spread it properly and evenly. The temptation must be avoided of thinning the enamel in order to facilitate the brushing. The enamelling process consists, first, of an under coating of enamel, which flows easily owing to the presence of turpentine, and, secondly, a finishing coat, which is less easily spread as it contains a somewhat large proportion of linseed oil and gum. The two coats of enamel take the place of some or all of the colour coats, no varnishing of any kind being required. The inside of the roof is usually enamelled.
For staining and varnishing the inside of the body a paste wood filler, having a base of whiting or similar material, is used. This is thinned with white spirit, a little gold-size being added, and is coloured with ochre, umber or other pigment according to the shade required, after which the surface is varnished. If the natural colour of the wood is to be preserved, a coat of linseed oil and gold-size allowed to dry and followed by one of a good elastic varnish will give the desired effect.
The bonnet is painted in the same way as any of the metal panels of the body, but the chassis itself does not need so many processes as the body. The crossbars and other parts of the Chassis which are hidden. when the body is mounted are given a coat of black. A coat of lead colour has usually been applied before the chassis is delivered. This is papered down and any designs of rust carefully treated, and the coat of priming made good. No protracted coats of filling-up are necessary, but a certain amount of stopping-up is necessary, after which another coat of lead colour is applied, then a colour coat and finally one of varnish.
The Cellulose System.
Up to the present the cellulose, or lacquer, process is used in this country almost exclusively for private cars, but its application to passenger and commercial vehicles is simply a question of time.
The advantage gained is the production of a surface which resists to a high degree the abrasions of dust
and dirt. Mud does not harm it if left to dry on, whilst tar, petrol, oil, salt air and other mediums which spoil any usual paint surface do not affect it. It is also highly suitable for painting a surface which is subjected to a certain amount of heat, such as the bonnet. It has been found, however, that if a non-freezing solution of alcohol and water used in the radiator be splashed over the bonnet it will attack the surface.
The nature of the cellulose coatings is altogether different from those of ordinary paint and varnish. With these, linseed oil is the ingredient which gives the paint its protective value, and turpentine is used to facilitate spreading and to assist drying. The ordinary paintwork, when finished, is always undergoing a very slow process of chemical change, chiefly oxidation. A typical lacquer is a solution of nitrocellulose in alcohol which has been treated with acetic acid, to which are added the pigment and the various agents giving elasticity and adhesiveness. The lacquer dries very quickly ; therefore, in order to cover a large surface with a thin and uniform coating, the material must be sprayed on. As may be expected, a quick-drying lacquer is also hard, so that, when building up the foundation, the question of elasticity must not be overlooked. The lacquer is not only highly resistant to nearly all materials which harm ordinary paint, but it is also chemically inert.
This newt kind of finish is chiefly criticized because of its lack of lustre, but it can be polished, so that, even if it does not have the lustre o a newly varnished job, it will usually be more presentable than the conventionally painted vehicle in the course of a few months. Cellulose painting is also criticized because cracks develop round the door plate and mouldings, and what are described as "orange peel" effects and white spots are set up. But these defects are usually the result of the lack of training or efficiency on the part of the painter and do not appear when skilled workers are available and the right material is used and the conditions of application are suitable. No paint film, however thick or elastic, can hold a loose screw head or remain unbroken on any part which is undergoing continual Strain, neither can it bridge a gap. Defects arise from similar causes with ordinary paint and varnish work, which process had been in constant use for many years before the inception of motorcars, but the process is not abandoned because of the defects; rather are steps taken to prevent the recurrence of these untoward incidents.
Cellulose lacquers producing a comparatively thin B39 film have been in use for some years past for various industrial purposes. Great improvements have been effected in the manufacture of aeroplane dopes, which are an allied class of protective coating. The use of a cellulose lacquer or enamel as a substitute for car painting has been made possible owing to discoveries which now permit a greater percentage of cellulose to be dissolved in the mediums used, whilst retaining a proper degree of fluidity to allow the lacquer to be applied to the panels, usually by means of a spray.
The priming coat of the cellulose process is often a red oxide pigment mixed with oil and varnish. This is, then, a coat having similar ingredients to that which might be used for the ordinary process of painting. This primer is used because it adheres readily to steel or aluminium panels after they have been properly cleaned, and has been found more satisfactory than using a true lacquer primer. This coat can be applied iith brush or spraying pistol and is made for drying at the usual paint-shop temperature, or the drying may he accelerated by baking. The stopping-up is also done with a composition which is made up largely by means 'of old formulas, and is applied with a knife.
The surfacing or filling coats may be brush or sprayapplied and have, again, in some instances, many of 'he old-fashioned ingredients. Those paint manufacturers who do not adopt a cellulose filler claim that the regular materials used for the undercoatings give better adherence, and no doubt something is gained in elasticity. On the whole, these undercoatings may be regarded as new preparations specially adapted for the superimposing of lacquer coats. These coats require about four hours to dry after each, whereas a lacquer filling coat dries in about half an hour.
After four or more coats of the filler have been sprayed on, the staining coat is applied, and it is rubbed down in the usual way with stone and water. When
perfectly dry, the next process consists of a sealing coat. This is sprayed on thinly so as to dry quickly and not attack the undercoatings, but, instead, seal them, for it must be remembered that cellulose paints act as removers of ordinary painting mediums.
The next cellulose coating is sprayed on more generously, with the pistol held closely to the work, the spray being controlled so as to keep it thinner at the edge of beadings and mouldings. Before the next coat is applied, the surface when dry is rubbed with a special grit paper and water, the third coat then being sprayed on. If the natural velvet lustre of the lacquer be, not required, then it undergoes a polishing process. The wax polishing compound is rubbed on by means of a cloth pad or with a mechanical mop.
Those parts which have to be protected from the spray, such as the windows, fittings and so on, may be covered with vaseline. Special compounds are also sold for the purpose. Parts not to be sprayed may also be masked with paper or cloth held by gummed strips, paste or elastic. This masking also gives scope for the use of two and three colour effects.
Lining and other ornamentation, which is done before the final coat of varnish with an ordinary painted job, is effected after the final coat of lacquer and before polishing. Either lacquer or the usual japan colours may be used, the latter being easier to work, but the lines themselves, if not done in lacquer, must be, pencilvarnished afterwards.
The equipment consists of a spraying chamber large enough to take the bus body on its mounting horse. This chamber is fitted with exhaust fans to carry away the fumes effectively. An electric motor drives an air compressor for generating the spray, the equipment including the hose and its connections, gauges, valves and regulators and one or more pistols or spray guns as required.