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CELLULOSE FINISH FOR COMMERCIAL BODIES.
Page 125
Page 126
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The Difficulties Standing in the Way ; How They are at Present Avoided and Finally will be Definitely Overcome.
By W. Fletcher Starkey.
CELLULOSE having established itself in the motorIL/car industry, the question naturally arises whether the same process of finishing coachwork is equally applicable to the commercial vehicle. For successful cellulose painting—and by this we mean cellulose applied by the spraying pistol, because brushing cellulose is certainly not yet a commercial proposition for large surfaces—certain features are essential for the production of good and durable work.
In the first place, wood is quite unsuitable—it is so prone to absorb moisture wherever the Weather can get at it and, hi consequence, cellulose will leave its surface much quicker than will ordinary paint and varnish. Cellulose, when dry, has a much harder film and is, therefore, more brittle, and its key or adherence to such a surface is less tenacious than the more elastic paint and varnish.
Most commercial vehicles are constructed of woodwork and, furthermore, have considerable ornamental beading of the same material. Whilst this method of construction obtains, it will not be possible to use cellulose with the same degree of satisfaction as paint and varnish now give. Experience proves conclusively that the vibration which is set up during the transit of a vehicle in its normal course will fracture the cellulose film very quickly at the junction of the beading and the main body of the vehicle. Then, of course, the weather will enter the woodwork and the work of destruction thereupon proceeds apace.
The Destructive Effect of Vibration.
There is no doubt about this. It is not by any means a theory ; it is actually what has happened and what will happen again whenever cellulose is applied to a wood surface subject to vibration, and particularly so where beading is present. The writer has seen railway coaches, tramcars, buses and commercial vehicles all go this way and, to make them good again, ordinary paint and varnish have always had to be resorted to,
Now, it would appear that, to secure the special merits which cellulose can supply, viz., speed of drying, hardness and durability of the film, there are two courses open. One lies in the direction of using metal bodies and beading and the other in developing the cellulose product further and further to suit the needs of the body blinder more effectively. No doubt ultimate success will be secured by the paint manufacturer and the coachbuilder A typical example of cellulose finishing of a van body, showing that m eet ing halfnames can be successfully written on cellulose surfaces if the right WIlY,materials and methods be employed.
To this end we shall do well to give consideration to the points that matter, so that we may know what it is that we have to overcome. It is recognized in the motor trade that smooth steel or aluminium is the best surface on which to apply cellulose. Considerable care is expended on the preparation of the surface to ensure absolute cleanliness. There are various methods of first-coating or priming the surface, and both oil and cellulose primers can be used. , It is an advantage in the cellulose process to stove the undercoaters, and this is another argument for using metal—it does not warp in the staving. Stoving the undercoaters makes them hard, tenacious and resistant to the cellulose coatings. When using oil primers it is always advisable to use the staving variety and to stove for four hours at a temperature of between 120 degrees and 140 degrees Fahr. The film will then be quite hard and will stand scratching with the nail almost without marking.
The Surface of the Primer Must be Flat.
To have any sheen at all showing on the surface of the primer when dry would be certain to produce some drying trouble in the later stages, because the cellulose is very penetrating and would soften such a film to such an extent that its character would be changed and it would very_ probably remain indefinitely tacky. Where cellulose primers are used this same trouble does not arise, neither is there the same need for staving, because cellulose will dry of its own accord in half an hour ; but it does make a special demand of its own. It will not adhere to doubtful surfaces quite in the same way as an oil primer. Let there be the slightest suggestion of grease on the surface and cellulose will not adhere; therefore extra care must be taken accordingly.
It is customary to heat the metal surface to a tem perature as high as it will stand, having regard to the woodwork there may be in the body of the vehicle. This heating process drives out all greasy and other d eleterious matter. The surface is afterwards washed down with a cleaning solution and then treated with a rustremoving pre
, paration a n d washed again• with hot water. It is then thoroughly drie ci and handled as little as possible prior to the application of the priming c o a t, which naturally• is applied immediately after the cleaning process. Cellulose priD63
mers dry in half an hour, whereas oil primers require the whole night in which to become hard. The former are more suitable for metal surfaces and are quite successful when proper regard is paid to the preparatory cleaning processes; the latter are better for wood surfaces as they have greater key and penetration.
Filling, stopping and final enamel coatings can be either all cellulose, or, if preferred the filler and stopper may be of the usual oil variety, on which the cellulose enamel will adhere quite satisfactorily. It is zn advantage to make such a selection of material for the bodywork that the surface requires the minimum amOunt of filling. In the cellulose process this promotes extra durability.
The method of application of the various coatings is essentially similar to that employed for finishing motorcars, with which we are all familiar. It is more important that reference be made to certain other points which do not apply in the same way when finishing a commercial vehicle. Features such as lining, writing, transfers and colour combination, enter more closely into this work and have to be dealt with specially. We will, therefore, take them in that order.
Lining and Embellishment of the Finished Surface.
It is still the most satisfactory way to use the regular coachpainter's oil-bound lining colour for lining. All cellulose material is too rapid drying to make effective lining a practical proposition. An attempt is made here and there to reduce the speed of drying by certain additions which certainly slow up the material, but at the same time they, unfortunately, soften it, so that the lining rubs up when the surface is being cleaned when in service, At the same time, the ordinary coachpainter's lining colour, which is more easy to apply, has, of course, to be protected with a suitable oil varnish. This, .again, is employing a softer and more elastic material than the true cellulose, so that the polishing or cleaning process, to which cellulose so easily lends itself, is rather interfered with. Yet a good flatting varnish will stand the rubbing involved reasonably well.
However, to be completely satisfactory the lining on a C211ulose job will have to be cellulose also. This will need further research into the material and, maybe, a special tool (rot a brush) with which to apply it— something like a fountain-pen filler—will be the solution.
The Writing of Names and Signs.
With regard to the writing of names, etc., the same remarks apply as for lining—excepting that writing most probably will always have to be accomplished by means of suitable hair brushes. To those who are not fully acquainted with the difficulty involved, we would explain that cellulose dissolves the preceding coating, and to attempt to write with cellulose would be to spoil the surface and to discolour the lettering, so that the job would look very smudgy.
Transfers, which are largely used where the volume of work justifies it, can be affixed to a cellulose surface quite easily, and by the usual method, and must be protected by a coating of oil varnish. Here, again, the uSe of an oil varnish rather spoils the job, as oil varnish will not rub and clean up with the same ease as cellulose and, furthermore, will not stand the same kind of wear. Again, the adhesive is inclined to remain soft, which renders the transfer liable to become easily damaged. The most effective way of applying a transfer o cellulose iS, first, to moisten the surface with cellulose thinners, then, while still wet, to apply the traps.; fer. It will adhere perfectly, but it is by no means an easy. operation.
Cellulose Varnish for Interior Work.
It is not advisable .to use cellulose varnish—or " clear " as it.is :More generally called--for exterior varnishing, because it deteriera,tes -quickly. The sunlight causes the film to become speckled, as it were, with faint White spots due to disintegration.
Where several coIourS are required in the decoratiOn of a vehicle, mostly this can be taken off by means of masking paper or. coninotind=--the adhesiveness Of which must be very weak, so that it can be easily
removed without-marking. •• The Use of Brushing Cellulose.
So far, that form of cellulose known as brushing cellulose is not very successful for large surface's, and neither is it as hard as the spraying cellulose. Naturally, the additions, which are made to give the material the requisite elasticity for brushing, take away from its ultimate hardness, so that it will not clean or polish in the same easy way.
Most shops have been, and are still, investigating the application of cellulose for commercial-vehicle work, and on many of the points referred to in this article research is in a much more advanced State than it was. Nevertheless, considerable further work is necessary before cellulose can displace its "older brother " in standing up to hard wear, facility for complexity, of detail, elasticity in moving parts, for instance. Yet cellulose, with its rapid drying, hard film and retention of polish, will always be pressing a claim for place; it can quite easily be a future development in this important industry.
The illustration pictures a typical example of the work done by one of the concerns specializing in cellulose finish, and, as its name indicates, the vehicle belongs to a firm having many such vans_ Those responsible' for maintenance desire to test out the merits of this new finish, and have accordingly had this van finished by Messrs. Carolac, of Cricklewood, with Proxcelin, manufactured by Lewis Berger and Sons, Ltd.
Many firms have similarly finished vans in service, and the finish is undergoing periodical examination, so that improvement can be made en the right lines.' As time pasfses we shall expect to see considerable develepmeat in cellulose finishing of commercial Vehicles.