The Half-hour Synthetics
Page 53
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This, the Last of the Series of Painting Articles, Deals with the Limited Uses of the Quick-drying Enamels
THE idea of an enamel which possesses the quick-drying qualities of cellulose and the gloss of four-hour synthetic, has fired the imagination of paint manufacturers and paint users alike, but in actual practice the uses of these half-hour enamels are very limited. On pressed-steel cabs and vans, the panels of which require no filling, the half-hour synthetics are admirable if used correctly, and in the hands of a competent spray painter should come from the gun with a good gloss which requires no polishing as does -cellulose.
For work which requires filling, however, the enamels sink a lot, no matter hOw well the fillers be sealed. When this sinkage,, takes place the job has either to be polished or varnished, in which case, of course, the cheaper four-hour synthetic enamels could have been used. Most half-hour nitrocellulose synthetic-resin enamels are more expensive than ordinary cellulose or synthetic enamels.
A Four-day Job
In this article we will deal with the case of a repaint job on a large van which .has previously been painted in four-hour synthetic and which is now required in half-hour enamels on account of a speedy finish being essential, four days only being available for the job.
The painting procedure will be as follows:—Examine the existing finish for bad cracks or complete paint failure and if all appears to be in order, rub down as for an ordinary repaint, ensuring the complete removal of the lettering. Mask the cab as necessary after removal of plated fittings such as door handles, and spray the complete vehicle with a coat of 'an oil-base redoxide primer formulated for both cellulose and synthetics; most paint firms make such a primer . The Use of a Sealer Allow overnight drying at least, and paper lightly, using wet-or-dry No. 280 C abrasive paper dry. Dust off well and the work is now ready for the half-hour process.
As a rule there are no undercoatings for the half-hour enamels, but a sealer coat is necessary, especially in the case of the repainting example with which we are now concerned. The sealer is thinned with cellulose thinners and the first coat should be in the form of a "mist coat "—a wet coat at this stage might lead to lifting of the primer and
the old finish underneath. A second coat of sealer is now Sprayed on in a slightly wetter condition and at a pressure uf 50-60 lb. per sq.• in. Allow a drying time of about 11 hours and then proceed to put the work into colour.
As it is a mistake to have too many
coats of half-hour synthetic enamel on the work, it will be necessary to organize the job so that three coats at the most will produce a. good finish. The best method is as follows:—Mix the half-hour enamel with its appropriate thinner at a consistency of 40 per cent. colour, 60 per cent, thinners, and spray on a fairly wet coat until coverage and good opacity are achieved. Whert hard, which will be in about three hours, flat with No. 320A paper used wet with soap as a lubricant, -and a ground of the correct colour is now ready for the final two finishing coats.
Using Two Sprayers The final coats are what are technically known as a "double-headed coat," and on a large job it is necessary for two sprayers to work together in the following manner:—One man has enough colour mixed for the whole job at a consistency, of 50 per cent. colour, 50 per cent. thinners, and the other has for his gun a material of 80 per cent, thinners and 20 per cent.
colour. When sufficient scaffolding— ladders and planks—has been erected, the first sprayer sprays on a good wet coat of his 50-50 mixture as rapidly as he can get round the job. As soon as there is sufficient space to work, he is followed by the second man with his 80-20 mixture.
This ensures a double coat in about a 15-minute interval, and provided the sprayers work rapidly and keep the edge of the areas open, the resultant coating should have a fairly good gloss which does not require polishing or varnishing.
Sometimes a vehicle to be sprayed is so large that it will require two men to put on one coat, and if the compressor be not large enough to run four guns (28 cubic ft. of air per -minute are required for four guns) the "following up" method has to be abandoned in favour of a different technique. This consists of dividing the job into sections and spraying accordingly. When the job is a tipper the simplest plan is to do the cab first in the manner detailed, allow to dry, and after covering with a dust sheet to proceed with the body or van portion.
Avoiding Overspray
Sometimes this sectional way of spraying may be aided by the removal of doors and wings, but whichever way is preferred, the object that should be borne in mind is to get the colour on the job in a wet coat and to avoid overspray.
On a half-hour enamel job the lettering and decorative work are carried out on the panels without flatting, and the lettering, transfers, etc., are-" pencil varnished," allowing in. overlap on to the panel.
The times and the various operations should be approximately as follows:— First day: two men rub complete job and mask for spraying; spray primer (18 hours), i.e nine hours each; second day: two men, dry-paper primer, spray sealers, spray solid colour and flat ready for finalcoats (20 hours); third day: two men spray final coats, black chassis, unmask, attend to details, and clean up ready for lettering (18 hours); fourth day: letter and pencil varnish.