An important consideration both in paint immersion and paint spraying is the number of coats of paint which should be given in order to produce the required appearance and ensure durability. This must necessarily depend upon the use to which the article to be painted is to be put, and it will also depend upon the appearance. In paint spraying it is clear that any thickness of paint required could be obtained by continuing the operation of spraying, but it has been found in practice that it is not desirable to continue the application of the paint beyond the point when every part of the surface is completely covered. It is a well-known fact that three thin coats of paint applied separately will prove more durable than two thick coats, even if the quantity of paint used in the two cases is identical. The same is true to a great extent when the paint is sprayed on. When it is applied by dipping another important element enters into consideration, and that is, thin coats are a practical necessity, because if an attempt was made to make the coat thick it would inevitably result in unsightly runs, fat edges, etc., which would wholly spoil the work. Where the object is only that of a temporary protection, as in the case of iron castings, a single coat will usually suffice, because the castings when fixed in position will be painted with the building in the ordinary way.
Sometimes a priming or first coat of paint is given by dipping, and the following coats are applied by brushes or by spraying. There appears to be some idea that when paint is applied by brushes it is forced into the pores of the wood and therefore holds better; as a matter of fact, in a well-designed paint the thinners will penetrate the wood and no particular force is necessary; but if it should be, it will be certainly supplied by the spraying process.
Advantages of the Dipping Process.
The advantages of applying paint, japan, enamel, or varnish by the process of immersion are obvious. The saving of time is the chief advantage, and this will be in most cases very considerable. For instance, a complete wagon can be painted by dipping in a very few minutes, while large and intricate agricultural machines may be painted in the same way in a fifth part of the time it would take to spray them, and probably a twentieth part of the time it would take to do the work by hand.
There is another advantage which should be mentioned when paint dipping is compared with spraying. In the latter, an outfit consisting of sprayer, air compressor, exhaust, cabinet, etc., are all necessary. In dipping, however, the apparatus may in many cases be of the simplest character. For example, in painting bedsteads all that is necessary is a narrow but deep tank containing the paint, into which the bedstead may be plunged by hand and then hung up to drip over a metal-lined floor.
Fig. 16.—A Morris Standard Electric Trolley Hoist.
In Birmingham, hundreds of iron bedsteads are dipped in this way every day. The head or foot of the bedstead is taken in the two hands of the operator, dipped into a tank of black japan, and immediately hung up on a travelling chair, which slowly carries it across the room to the oven, the superfluous paint dripping off during the process.