As already mentioned, many firms who use the paint dipping process buy their paint in paste form with the requisite amount of driers added, and thin it themselves with white spirit, which is a petroleum product specially prepared for the purpose. Turpentine is, of course, too expensive, and the spirit costs, under normal conditions, little more than one-third of the price, although exact figures cannot be given owing to the fluctuation in the price of these materials. Some manufacturers use a white spirit, to which has been added from 25 to 30 per cent. of genuine American turpentine. This, of course, renders the mixture somewhat expensive, and possesses no advantage excepting that it gives the characteristic smell of turpentine; but this, it appears, is not important in a factory, and the pure white spirit is quite suitable in itself.
According to Mr. S. Roy Illingworth, A.R.C.Sc., A.I.C., B.Sc., Lond., expert to the Gas Lighting and Improvement Co., Ltd., turpentine substitutes were first placed on the market about 30 years ago, and consisted at that time of a mixture of turpentine and kerosene oil. Investigations led to the production of white spirit, drying as quickly as turpentine, or even quicker if desired. Several grades of this material are made, varying in flash-point from 80° up to 150°. The flash-point of genuine turpentine, it may be mentioned in passing, is 90° to 91°, and the 90° flash-point spirit is the one usually employed for painting purposes, although the 80° flash-point would answer equally well for dipping. The best grade is practically free from smell and is suitable for admixture in all paints, excepting those having a bitumastic or asphaltic base, or paints in which very common driers containing cheap rosin are used. A rough-and-ready test as to the quality of white spirit is to dip a piece of blotting-paper into it and hang it up to dry. In the course of an hour-and-a-half it should have wholly disappeared, leaving no smell or stain behind it. If there should be a stain the sample should be viewed with suspicion. Another test of the same kind is to drop a little spirit on a piece of white writing paper and to leave it for three-quarters of an hour, when it should have disappeared without leaving a stain. In addition to these tests it might be advisable to mix a little spirit with paint and to try it experimentally.
In the evaporation of turpentine it is usually admitted that a small proportion, say, from ·25 to ·50, is left behind owing to oxidation. In the case of best white spirit, however, very little residue will be left behind, certainly not sufficient quantity to interfere in any way with the drying. The use of the spirit has proved so successful that practically nothing else is now employed in paint dipping. It is, however, important, of course, that the best quality white spirit be used, but as this is so much less in cost than turpentine, there is no temptation to use an inferior quality.
The Quantity of Paint Required for Dipping and Spraying.
Speaking generally, rather less paint will be required when it is applied by dipping, or spraying, than is necessary when it is applied with a brush. But there are many exceptions, particularly in spraying, where it may be necessary to give a coat considerably thicker than usual.
The following table will be found useful as a guide, but it should be remembered that the actual space covered will vary considerably with different grades of paint, so that the table should only be taken as approximately correct. In cases of doubt, careful calculations should be made after a given quantity of paint, varnish, or lacquer has been applied to a surface of known area. This will serve as a useful guide in ordering in the future. It need hardly be said that the quantity of paint necessary will vary largely with the surface to which the paint is applied. For example, an absorbent surface, such as unprimed wood, will require much more paint than metal work, which absorbs little or none.
Spreading Capacity of Paints.
| Per Gallon on Wood. | Per Gallon on Metal. | |
| Priming Coats | 60 to 70 | 90 to 100 |
| Mixed Paint 2nd coat | 85 to 90 | 100 to 120 |
| Varnish (on paint) | 95 to 110 | 120 to 130 |
| Lacquer (on paint) | 100 to 115 | 130 to 150 |
| Enamel (on paint) | 75 to 80 | 85 to 95 |