Enamel and Lacquer Finishes in the
DURING the last few years enormous progress has been made in the art of finishing, particularly in connection with enamels and lacquers.
The name of Ericsson has always been associated with the highest quality finish, and this reputation, of which the Company is rightly proud, is held as a result of continuous research and development. Wherever possible practical tests as well as laboratory investigations have been carried out, and this policy has proved of particular value and materially assisted in the perfecting of enamels and lacquers especially suitable for tropical conditions. Many hundreds of samples have been sent out for exposure in countries where climatic conditions are severe, and engineers abroad have given wholehearted assistance and valuable suggestions which have considerably added to the progress made.
In view of the highly specialized nature of the requirements of the telephone industry and after considerable investigations extending over a number of years, the Ericsson Company installed a modern and highly efficient plant for the manufacture of enamels, lacquers, varnishes, etc. The enterprise has been a success and large quantities are now supplied to other firms.
Much space would be required to give full details of the various classes of finishing materials used, but a general description of the most important will no doubt be of interest.
It will be admitted that from the point of view of appearance, well finished woodwork cannot be equalled. The materials used may be divided roughly into two classes, i.e. stains and fillers for the preparatory processes and varnishes for producing the smooth protective film over the surface.
Stains are grouped as follows:-
1. Water Soluble Stains
These include chemicals such as potassium bichromate or permanganate, extract of logwood, ammonia solution and of greater importance aniline
dyestuffs. The resulting colours are very fast to light and do not fade perceptibly on continued exposure. The great disadvantage is that the water swells the wood and additional sand-papering is therefore necessary before further operations. There is also a danger of warping unless the liquids are applied carefully, so that this class of stain is rapidly going out of favour. The purely chemical stains are unpleasant to handle and the range of available colours is small.
2. Pigment Stains
It is possible to grind certain normally opaque pigments to a very fine powder, which when suspended in a suitable liquid medium may be applied to wood in a very thin film so that a semitransparent effect is given. The colours are very fast to light but there is a tendency somewhat to obscure the grain of the wood. Considerable skill is necessary to apply them successfully and this limits their use to a great extent.
3. Spirit and Naphtha Stains
The simplicity of application combined with the
wide range of available colours has resulted in the widespread use of these stains which consist of dyes in various solvents such as alcohols and coal tar distillates. They may be applied by a cloth, brush or spray and dry rapidly without swelling the grain of the wood. The fastness to light is, generally, comparatively poor so that these stains should be selected with great care. Much progress however, has been made recently and it is now possible to obtain a range of colours which are equal in non-fading properties to the water soluble variety.
As a general rule staining is the first step in wood finishing and although spirit and naphtha stains may be applied after the filler, water stains must be applied before. The art of selecting and applying a suitable colour and depth of shade controls the final effect to a large extent. The next step is the filling process, the object of which is to fill up the pores or grain of the wood and thus produce a smoother effect when the varnish is applied.
Wood fillers are made in the form of a paste and contain a high proportion of solid matter finely ground in various liquids. The latter are usually of the linseed oil type because the application is simple and smooth for the operators. They are applied to the wood with cotton waste using a circular motion, and the excess is wiped off in a direction opposite to the grain, leaving the pores filled with the paste. They are generally coloured with pigments which assist the staining process and form an excellent base colour. A useful range is light oak, dark oak, walnut and mahogany.
A cellulose medium may be used in place of oil if rapid drying is desired. The former will dry within an hour or so, but cannot be applied with the ease of the latter which should be left overnight to harden.
The final and no doubt the most interesting step is the varnishing and finishing-off processes. These may be considered in their historical order and the first and best known is “french polishing.” This consists of pure shellac in denatured or wood alcohol, with small quantities of other gums added to improve the gloss and flexibility. It is applied with a rubber, i.e. a cotton-wool pad moistened in the liquid and enclosed in cloth or chamois leather. This is rubbed over the surface in a circular motion and numerous applications with constant, skilled labour are necessary to obtain a first class finish. A little linseed oil is added to the pad to facilitate working and to impart flexibility to the film when fully dried. The process is not used to any great extent for mass production work owing to the high cost and time required, furthermore it is tedious and needs much experience.
A modification is now taking its place in which the shellac varnish is first applied by a brush or spray and the final operations only are carried out by the true
French polishing method. The resulting finishes in either case are generally excellent but are costly, lack resistance to moist heat and if no oil is used there is a tendency towards brittleness.
A later development and one which has been widely used during the present century is the oil rubbing varnish or piano type finish. in this case a varnish made from linseed oil combined with gums of the copal type forms the basis. These are thinned with turpentine to a working consistency and applied by a brush or spray. Two or three coats are required and a drying period between the coats of up to two days is necessary. The appearance at this stage is far from attractive so that the surface is
made smooth by hand finishing and flatting processes, the most common method being to rub the varnish with a flat felt pad to which a mild abrasive such as wet pumice powder is applied. A high polish may be attained by following the pumice powder with milder abrasives such as rottenstone or special polishing liquids. Providing a first class varnish is used the finish is extremely durable and has the advantage that it can be carried out with little or no equipment. Also, the risk of fire is remote so that this method is ideal for the repair shop where the labour is mostly unskilled. This process is more rapid than might be imagined and gives a very fine effect.
During the last few years a type of varnish has been developed which is comparatively rapid in drying and at the same time produces hard and flexible films. It is dry to the touch within an hour or so after application and may be flatted after from four to twelve hours. The main advantage of this process is the freedom from sinking as the film hardens. This is because the oil dries by oxidation and has more tendency to expand than contract. Due to this fact and also the high degree of moisture resistance, the process is still used by many manufacturers.
A still more recent development and now the most important class of finish is the cellulose type. It can safely be said that provided high grade and suitable materials are used, results can be obtained which are fully equal to those given by any of the other methods. The features of cellulose films are the high degree of hardness and flexibility with excellent resistance to wear. Water, acids and common solvents such as alcohol or turpentine do not affect the finish.
The basis of this varnish is nitro cellulose which is combined with gums,
plasticisers and other products to improve or modify the effects given. A wide range of solvents is available, which differ as regards drying speed, flashpoint, odour, flowing out properties and toxicity, and are selected to meet the conditions of application.
Very little cellulose varnishing is done by brush owing to the difficulty in applying more than one coat by this method. Spraying is in general use and most woodwork is now finished in this manner.
There are two general processes used in cellulose finishing. The most inexpensive and rapid is the direct method whereby two or three coats are applied and allowed to dry for an hour or so between coats. This is suitable for internal work or where protection is desired without high class finish.
The more expensive and extremely fine results are obtained by spraying on two to four coats which when thoroughly hardened are flatted with a pad and the usual abrasives and finally burnished to the required degree by polishing compounds. Mirror-like finishes may be obtained by this means. Another interesting method is that known as the “bodying-up process.” For this a special liquid is used which softens cellulose without actually dissolving it. The liquid is applied to the dry varnish in a manner similar to
French polishing so that the varnish is softened and forced into the grain of the wood, producing a smooth lustrous surface.
Many other methods of cellulose wood polishing are in regular use. To those who desire a range of finishes suitable for various types of wood and qualities of work, the cellulose processes are indispensable.