More than two hundred different grades of varnish are made for various purposes, but perhaps not more than a quarter of them are used by house painters.
For the decorator’s purpose, they may conveniently be divided into two main classes – oil and spirit varnishes, for although there are other varieties, they find but little application in the painting trade.
The decorator is mostly concerned with oil varnishes which may here, for the sake of convenience, be classified under two chief heads – those for outside and those for inside work; each embraces several types. The primary consideration, in exterior varnishes, is their ability to withstand the weather and they must, in consequence, be both hard-wearing and elastic. These properties are imparted to them by employing hard resins with a large proportion of oil to gum. For interior varnishes, on the other hand, elasticity is not so important and the percentage of oil is consequently substantially less. The following are notes on the leading varieties.
Coach or carriage varnishes are not often used by the painter, but they deserve to be mentioned as representing probably the finest examples of the varnish-maker’s art, combining, as they do, a high degree of gloss with great resistance to exposure. They are made from selected hard resins and the purest-quality oil and are generally slow-drying. When used as the old coach-painters employed them, over a groundwork prepared with infinite care and properly flatted undercoats, they give a beautiful and lasting finish though one which, by reason of the cost and labour involved, is seldom attainable by the present-day painter.
These are similar in composition and character to coach varnishes and, at their best, only a little inferior in quality.
Yacht or Boat Varnishes
Varnishes of this type are primarily formulated for use on boats or ships or where conditions of great humidity prevail, which would tend to turn ordinary oil varnishes white in a short time. On account of its moisture-resisting properties, china wood oil is largely employed in their manufacture; a good yacht varnish should be able to withstand prolonged immersion in water without more than slight signs of whitening, loss of gloss, or deterioration. It can advantageously be used in house decoration in such rooms as kitchens and bathrooms, where there is usually heavy condensation; the extra cost will, in most cases, be fully justified by superior durability.
The so-called oak varnishes are widely used by decorators and the description covers a considerable number of grades. Broadly speaking, it may be said that the term covers the ordinary run of inside varnishes, made from comparatively inexpensive resins, though some of the more elastic varieties are suitable for external work. Such names as ‘ copal oak,’ ‘ hard copal,’ or ‘ pale copal,’ denote as a rule varnishes of a rather better quality than ordinary ‘ oak.’ ‘ Church oak ‘ is the description applied to a type of varnish which dries with a hard film, which does not soften when subjected to warmth, and is thus suitable for application to seats and pews.
Flat Varnishes: Flat or egg-shell flat varnishes are intended only for interior work and are made by taking a quick-drying varnish, adding extra thinner, and incorporating a matting agent, such as paraffin wax or aluminium stearate. They should be applied only over a coat of full-gloss varnish and, as they are somewhat deficient in flowing properties, it is usually necessary to stipple the coating.
If aluminium stearate has been used to impart flatness, a film of flat varnish can be recoated, but if wax has been employed, there is a risk that a gloss varnish subsequently applied may fail to harden properly. The nature of the matting agent can generally be determined by rubbing the film with a soft cloth; if wax has been incorporated, friction will result in a slight gloss which, however, will not occur if aluminium stearate or some other waxless ingredient has been used.
These should not be confused with flat varnishes and it would be better if they were referred to by their alternative description, ‘ rubbing’ varnishes. They are intended to be used as undercoatings to the final coat of varnish and are formulated to dry quickly to a hard film which can be flatted down. They contain, therefore, a high proportion of resin and relatively little oil. A good rubbing varnish should be capable of withstanding, without injury to itself, water or oil employed in the flatting operation and should not soften or gum up under friction.
Varnishes of this type are designed for use not as undercoats or finishes, but to be added to paints to impart hardness, improve the gloss, and promote the flow. While some ordinary varnishes can successfully be used for this purpose, not all are suitable and consequently it is always safer to employ a special mixing grade. The latter should be pale in colour, should not react with basic pigments, such as white lead or zinc oxide, and should be miscible with paints or enamels without causing curdling or precipitation.
These are not extensively used to-day, for, although they are capable of giving a rich and lustrous finish, the cost and length of time involved are in most cases considered too great. They are short-oil varnishes with a high proportion of resin and dry reasonably quickly. At least three coats are necessary, each being given plenty of time to harden before being carefully flatted. When the final coat is hard, the surface is rubbed with felt and pumice powder until it is dull and then treated with a mixture of rottenstone and linseed oil to bring it up to a semi-gloss. After being cleaned off and wiped with a little oil, it is finally burnished with a soft cloth and Vienna chalk to a finish which, if the work has been properly done, rivals that of French polish.
Ordinary varnishes applied to floors provide a coating which, if subjected to normal wear and tear from being walked upon, soon crack and disintegrate. Special varnishes for floors are made from hard resins which possess toughness and flexibility and should also be sufficiently moisture-resisting to withstand periodic washing over a reasonable length of time; they should be applied in thin coats and be given ample time to dry and harden before the floor is used.
This useful material serves a number of different purposes in decorating and painting. It is made with a high percentage of resin and drier and a minimum of oil and, as it dries quickly, is often employed as a siccative. It is used also as a foundation coat and filler on wood and as a first coating on impervious surfaces, such as hard-faced plaster or glass. Owing to its low oil content, it gives a somewhat inelastic film and consequently should not be added to paint in excess or it may induce cracking. For a similar reason, it should not be applied over an oily or flexible ground.
This name is given to types of varnishes primarily formulated for application over wallpapers in rooms such as bathrooms or kitchens. The two main classes are French oil varnish and crystal paper varnish.
French oil varnish is, as a rule, a long-oil varnish made from pale resins. It is usually somewhat darker than crystal paper varnish, though special pale grades are obtainable suitable for application to delicately tinted papers. Crystal paper varnish is a spirit varnish. It dries quickly, with a brilliant initial gloss, and is procurable almost water-white in colour so that it can be applied over any kind of paper without altering the colour effects, but it is liable to bloom easily, soon loses its gloss, and tends to yellow and disintegrate fairly rapidly if the conditions to which it is exposed are severe. It is employed extensively for ready-varnished papers, but cannot compare in durability with a good-quality French oil varnish.
Storage of Varnishes
The present-day tendency in the trade to buy finishes only as and when needed, instead of purchasing them in bulk as formerly, has resulted in less care being paid to the conditions in which materials are stored. It should be emphasised that varnishes are particularly easily affected by sudden changes of temperature, and more defects than are realised at the time are due to this fact being ignored. They should be kept in a warm, dry place with a temperature maintained throughout the year, if this is practicable, at from 65** to 70 degrees F.
Varnishes should – and, in most cases, do – leave the factory in such a condition that thinning by the painter is unnecessary. If, however, the tin is not air-tight or the material is kept in unsuitable conditions, a certain amount of thickening up may take place so that application is difficult.
Present-day factory-made finishes, including varnishes, are complex and scientifically formulated mixtures, and any unauthorised addition may upset their balance and lead to premature failure and, it is well to remember, the manufacturer may disclaim any responsibility for any such failure if ingredients, which have not been specifically recommended for the purpose, have been added by the painter. Should a varnish prove too thick to apply, the proper course is undoubtedly to consult the makers. This, however, takes time and most decorators simply add turpentine; the risk of doing this is that some of the solvent may be trapped beneath the film when the latter begins to set and this may lead to trouble in the form of pin-holing. A safer course is to take some of the thick varnish, add to it about twice its volume of turpentine, mixing well together, and use this as a thinner. It should be understood that this practice is suggested only as one less likely to have an adverse effect than that of adding turpentine direct.
On no account should oil be added to a varnish.
Application of Varnishes
Those who remember the old days of coach finishes, before the introduction of cellulose enamels and varnishes and the spray, will recall the wonderfully high standard of workmanship achieved in the painting and varnishing of vehicles of the period. Such results were obtainable only by fine craftsmanship, the use of high-grade materials, and the most meticulous attention to every detail contributing to the success of the finish.
It is, of course, not practicable for the decorator to attain such a standard since very little of his work can be done in the paint shop where atmospheric conditions can, to a certain extent, at least, be controlled: nor will the cost of most jobs permit the careful preparation of the surface or the number of coats necessary to build up a perfect and durable finish. He must take things much as he finds them, though, by exercising a little additional trouble, he can get better results than are commonly obtained.
Cleanliness is an important essential. Not only the surface under treatment but the clothes and person of the painter and, above all, the brush he uses must be as free as is humanly possible from dust and dirt. A varnish brush should be used only for varnishing and, when not in service, should be kept in a properly designed brush-keeper: for the best work, a brush which has been well broken in is necessary. The surface should be properly dusted before any material is applied, remembering that corners, mouldings, and crevices usually conceal loose dust which, if it is not removed before the work is started, will be taken up by the action of the brush and spread over other areas where it will be more conspicuous.
Before commencing the actual application, make a careful examination of the surface to determine where to begin and finish, so that full advantage can be taken of mouldings, joints, and other interruptions in the continuity. Since varnish flows out by itself to a level surface, the chief consideration is to ensure that it is evenly distributed, without runs. The can should not be submitted to unnecessary shaking or air bubbles may result which are liable to give trouble. A clean wire stretched across the centre of the can is preferable to wiping on the sides, should any wiping be necessary.
The brush should be fairly fully charged, applied to the surface at the top and drawn down vertically, the pressure being progressively increased as the material is used up. The next stroke should be laid alongside the first and the varnish blended with what has already been applied and so on, until the section under treatment is completed. The work should then be gently but firmly crossed to spread the material evenly and finally the vertical strokes should be repeated with a still lighter touch.
Finishing coats should, as a rule, be flowed on a little thicker than undercoats, but in any case varnish is better put on too thinly than the reverse. The setting point varies not only with different types of varnish but with different varnishes of the same type. It does not depend on the consistency of the material, and it will be found that some varnishes, though they are rather on the thin side, yet take on an initial set sooner than others which are more viscous.
The resins used for spirit varnishes include shellac, rosin, spirit-soluble manila, damar, and sandarac; various types of solvents are used, but the two principal are alcohol and turpentine. The process of manufacture is simple, consisting of dissolving the resin in the solvent, generally by churning in revolving wooden barrels.
The spirit varnish most useful to, and most widely employed by, the decorator is shellac varnish. The proportion of shellac will vary according to the purpose for which the product is intended. Thus, for all-round decorative work, approximately 6 lb. shellac to a gallon of methylated spirit would be suitable, whereas for knotting, from 2J to 3 lb. per gallon would be adequate. In addition to its use as a sealing coat, shellac varnish is sometimes employed as a filler for hardwoods, for which it is excellent. In the past, it has been regarded as having only poor resistance to moisture, but improvements in its preparation in recent years have, to some extent, overcome this weakness.
Those spirit varnishes in which turpentine is the solvent are made principally from damar, mastic, or sandarac, with or without the addition of rosin. Damar is chiefly used for crystal paper varnishes, while mastic is the resin generally employed for the so-called ‘ picture ‘ varnishes, used by artists.
Spirit varnishes – especially those in which the solvent is alcohol – dry quickly, which makes them difficult to apply on surfaces of any size. A large soft brush or mop should be used for the purpose and the material brushed on quickly and in one direction: very little crossing is possible, owing to the speed of setting. Although they give films which are more or less impervious to moisture, they are useless for exterior work since they have little flexibility, soon become brittle, and are readily fractured by abrasion. As ferrous metals will cause them to darken, they should never be stored for any length of time in metal containers unless the interior of the latter is lacquered with a coating which will resist the solvents in the varnishes.