The decorator is sometimes required to paint upon shop windows, fascias, or other glass surfaces, and although this kind of work does not come under the category of general house painting, a few notes on the process are included in this post as being likely to prove of service.
It should be pointed out that by painting on glass is meant, in this instance, the type of work in which the decoration is carried out on the back of the glass and viewed through the latter; it differs from other forms of painting in that the first coat is actually the one which is seen, instead of being concealed by those succeeding it; thus, to a certain extent, normal painting practice is reversed.
The chief difficulty, as may be imagined, is to obtain perfect adhesion throughout to the smooth, impervious surface of the glass; this is particularly important, since any undue shrinkage of the paint film, which causes it to lose its grip, is likely to lead to more extensive peeling than would occur on surfaces of a different nature.
The glass must be absolutely free from any suspicion of dirt or grease; though it may look clean enough, it is capable of retaining a film of impurity, almost invisible to the naked eye, but sufficient to cause trouble if it is not removed. A solution of sugar soap, or one of pure Castile soap, shredded into hot water should be employed, and its application should be followed by rinsing in clean cold water and drying with a leather; as an additional precaution, a wipe over with a rag dipped in turpentine is to be recommended.
The paint used for application to a glass surface needs special consideration. It should be borne in mind that protection does not enter into the matter; what is chiefly wanted is that the paint should adhere well, obscure adequately, and go on evenly. Both the pigments and the vehicle employed must be chosen with these purposes in view.
It is important that as thin a coating as is sufficient to provide the necessary degree of opacity be employed. The thicker it is, the more it will contract and the greater the risk of the film pulling away from a surface which offers no mechanical key. The first coat, which, in this instance, is the main colour coat, should therefore be made from pigments which have great natural opacity and the desired colour obtained by the addition of stainers with intense tinting properties; by this means the minimum amount of pigment need be used.
Of the white pigments, titanium is the most opaque, with lithopone a good second. Of the coloured pigments, Prussian blue, vermilion, the red oxides of iron, chromium oxide, and chrome yellow have good tinting powers and should provide a range sufficient for most purposes. If pigments which have only weak obliterating or staining properties are used, a greater proportion of them will have to be employed so that the film will be thicker, and there will be greater risk of flaking taking place.
The vehicle must be of an inherently tenacious nature, and some of the proprietary paints specially formulated for use on glass use a high proportion of hard fossil gum or a suitable type of synthetic resin for the purpose. Raw oil should be avoided, and although boiled oil is rather better it is probably more satisfactory to employ a high-grade ready-bound colour, thinned with equal parts of good-quality mixing varnish and turpentine for the first coat on glass. Paste colours in oil should not be used, unless sufficient gold size is added to speed up the drying and counteract the softness of the film.
Even if the right type of pigments is used, it is unlikely that a single coat will obscure properly, and a second will be necessary. No attempt should be made to obtain the required degree of obliteration by applying the first coat, or any succeeding coat, thickly. On glass, as in practically all painting, two thin coats are much better than one. In applying the second coat the rule of coat sequence should be observed by making it rather more elastic than the first.
In selecting pigments for paint for glass, those fast to light should be given preference; though ordinary window or plate glass cuts off or absorbs a high proportion of the ultra-violet rays from broad daylight which are responsible for fading, sufficient active rays penetrate to cause loss of colour in paintwork exposed, even behind glass, to the sun. In cases where an exact colour or shade is required, allowances should be made for the fact that glass usually imparts a greenish tinge, and that unless the colour decided upon happens to be a blue or a green, there may be some distortion, although the extent of this will depend on the type and thickness of the glass.
Many failures of paintwork on glass are due to the presence of condensed moisture, particularly in shops in cold weather when the premises are over-heated and under-ventilated. The moisture thus deposited attacks the paint film from the rear, causing it in time to disintegrate and gradually to flake, the process being accelerated if any fracture occurs in the coating through which the moisture can penetrate.
Obviously, the best way to prevent this form of attack is to improve the ventilation, but this is a structural job which does not come within the scope of the painter’s work. If it cannot be carried out, the application of a good moisture-resisting backing coat is the most effective form of protection. Ordinary varnish is often employed as a backing for paint on glass, but is not calculated to stand up for more than a limited period to repeated water-deposition. The type sold as ‘ boat ‘ or ‘ yacht ‘ varnish, though more expensive, should be used if the price permits, and it will probably be, in any event, more economical to employ it in the long-run. In applying the varnish to the back of painted lettering or similar work, where the paint does not cover the whole of the surface of the glass, the material should be taken slightly over the edge of the paint on to the glass itself so as to seal the edges.
Failure frequently occurs, also, when the underside of sky-lights or roof-lights is painted to prevent too much sun from entering the room. This is due to the fact that glass is a good conductor of heat and is subject to a great deal of expansion and contraction according to fluctuations in the temperature. The use of good-quality, elastic coatings, applied as thinly as possible, will substantially decrease the risk of flaking.
It frequently happens that when a room with a tiled hearth is redecorated, the colour of the tiles is out of harmony with the new colour scheme and consequently must be changed.
The tiles should first be thoroughly washed with a solution of soda and water or with a good sugar soap, rinsed off carefully with cold water, and allowed to dry. Then take equal parts of gold size and American turpentine, mix them together and apply to the tiles by means of a piece of clean linen, folded over to make a wad. Only a thin film should be deposited.
When the mixture is dry and hard any suitable paint finish can be given.