First Stages in
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PAINTING A VEHICLE
As an example of a typical painting job we will take a drop-sided lorry on a chassis equipped with a coachbuilt forwardcontrol cab, and we will assume that the vehicle has been newly delivered from the coachbuilder in works primer.
Delivery is often quoted as being 'in works primer" and this is a point to watch; the present-day need for production tends to make the priming, which is the last stage of the work before delivery, somewhat of a hurried process and what should be the most important coat of paint is frequently left to unskilled labour. It is useful to remember that the primer coat is the only coat of paint that is in direct contact with the structure of the vehicle, whether it be of wood or metal.
Check the Primer
Before commencing wo rk a thorough examination of the paint then on the vehicle should be made. Look for the following points:—(1) On wood: primer standing up in ridges, called " ropiness," indicating that the paint has merely been [aid on thickly instead of, as is proper, brushed well into the grain of the wood; flakiness, due to application over damp wood, for instance, wood that has been out in the rain prior to painting; a powdery look, which is the result of a poor-quality primer -standing too long before finishing coats are applied.
(2) On metal of cab and ironwork of body: rust appearing through the paint, showing both insufficient cleaning and low-quality primer; " ropiness," due to heavy, careless application. It is, of course, essential to look for places that have not received any paint at , all—favourite " misses" are under cab edges, under and over door edges, and behind door shuts, in fact, any place not readily visible to casual observation.
Second Priming When a number of these faults appears in a primer job it will be necessary to reprime after thorough preparation. Where the primer appears to be in good order the same preparation is necessary in a modified form.
The body should be well rubbed down with rough abrasive paper of grade 80D (wet or dry carborundum paper), which will remove loose c6
It is essential to work clean and to wash off each section of the work as it is finished. The backs of some coach-built cabs are made of a pressed board, such as beaver board, and it is not advisable to rub these wet until several coats of filler have been put on; in the first instance, thenoall that will be necessary for the back of the cab will be a dry rub to
remove dirt, grease, etc. '
The removal of plated fittings, such as door handles, grab handles and mirrors, may take place at this stage, although some prefer to do this before commencing any cleaning work. When more than one vehicle is being painted at a time it will be found necessary to have a method of ensuring that the correct parts go back on the right vehicle and do not get lost.
If the vehicle is to have any coat of paint sprayed on, then masking is the next operation. This involves the complete protection of glass, chromium-plated radiators, and the like, with masking tape and stout paper. Neatness of working is essential, as nothing looks more slovenly than places which are unpainted due to a ragged piece of tape having covered up a portion of the work during spraying.
If the vehicle is to be brush-painted throughout, then all that is essential is that lamp rims and the edges of radiator bright parts be masked with a single strip width of masking tape Whilst not encouraging carelessness, this enables the painter to obtain a clean coat without having to worry about cutting it too carefully.
Ideally, for woodwork, the priming coat ought to be brushed, as the weight used in wielding the brush forces paint into the grain and ensures complete adhesion of subsequent coats. There are, however, newly formulated paints employing a medium which has high adhesis e qualities, thus minimizing any risk of paint flaking: Brush Marks On the body, brush marks do not matter, as the filler has to be applied and,,later, rubbed, but when priming the cab a light touch of the brush is• desirable, because filler should not be necessary and tiny brush marks in the primer will have to he removed by flatting.
All primers, other than those on a white-lead base, are safe to spray, and thinning should be done according to the maker's recommendations. Air pressure should be 50-60 lb. per sq. in.
Where a completely synthetic finish is contemplated, it is best to use a synthetic primer, although -a good, quick-drying, oil-base primer, if well brushed out and allowed time to harden, ought not to" work up" under the strong solvent action of the synthetic resin undercoats or enamels. The colour of the primer on the body is immaterial, as filler is to be used, although, obviously, if the finish is to be cream, then the primer ought to be a light grey in view of the fact that the cab will have no filler.
Overnight Drying • Overnight drying is preferable, although with a wet on wet-spraying process this is not possible (more will be said about these processes in a later article).
Oil-base fillers consist of slate powder ground in a varnish medium and are best brushed for heavy coatings, although they may be sprayed. When applied by brush the first coat should be laid off lengthwise, the next coat crosswise, and so on, alternately until the requisite number of coats is completed. Where the last
coat is brushed on and dried with some time to spare before ceasing the day's work, a guide coat may be sprayed or brushed on; this will have the advantage of leaving the work ready for rubbing first thing in the morning.
A good guide coat is made by mixing vegetable black with gold size to a smooth paste and thinning to a wash consistency with subturps. The object of the guide coat,
of course, is to facilitate rubbing7in, for when all the guide coat has been removed, the filler should be level,
Before Commencing to rub it is as well to stop tip screw holes on the cab and defects, i.e., deep dents in the body, with synthetic stopper. The rubbing of a normal job is done with 180C paper, used wet, and with. the paper folded •over'a rubber block to ensure uniformity of rubbing and a level surface. •
When all the rubbing is completed, accumulations of filler in the corners of ironwork should be scraped out and dry-papered clean. After a wet operation on a surface of such a porous nature, a lengthy drying time will obviously be necessary, and wherever possible this should be overnight, prior to application of the colour, which is the following stage in the work and which will be described in the next article.