THE object of varnishing woodwork, whether painted or not, is to give it a glossy, hard surface which dust will not cling. To, and therefore is easily cleaned, and to exclude damp. Being transparent, it does not conceal what it is laid over.
The varnishes most commonly used for woodwork are called oil varnishes, because they are composed of resins – copal, shellac, kauri, etc., dissolved in oil, usually linseed oil. They harden slowly, since the oil takes a long time to combine with oxygen, but when hard provide a tough and durable covering.
Oil varnishes fall into two cl;g3.s: outside varnishes, which will itaid exposure to the weather; and softer inside varnishes, which are suitable for indoor use only. Outside varnish is used indoors as well as out, and the finest kinds of varnish, such as pale carriage varnish, belong to the outside category.
Cheap varnishes are as a rule a very poor investment. Many of them will net harden properly, and in damp weather turn white and cloudy.
Spirit Varnishes, which here need only just be referred to, are resins dissolved in alcohol, turpentine, or other volatile oils. As they dry very quickly they are difficult to apply evenly, and may be very brittle when hard.
Oil varnishes improve with age, provided that they be stored in air-tight containers. Varnish kept in an imperfectly stoppered can will turn thick and be useless for good work. Since high-class varnishes are expensive, the corks protecting them from air should be beyond reproach. It is a good move to dip their heads into melted paraffin wax to seal the pores.
Three donts: Dont add anything to varnish as long as it is in good condition. Dont pour varnish that remains over after completing a job back into the unused varnish; store it in another can and use it for rough work. Dont pour varnish into any but absolutely clean vessels.
If varnish becomes too thick for use, it should be warmed in a pan of hot water, and thinned by the addition of fresh varnish (also warmed) and some turpentine. On no account should the mixing be done cold.
Surfaces for Varnishing
Varnish tends to show up any imperfections in surfaces to which it is applied. Any number of coats will not conceal roughness in the wood below. If varnish is to be laid on bare wood the wood should be made as smooth as is possible with the scraper and fine glasspaper and have its pores closed by rubbing it over with a wood-filler.
Painted work also requires careful smoothing. Where varnishing is intended, the last paint coat should be flat, since varnish works badly on glossy paint. And the paint must be thoroughly dry before being varnished, as otherwise it is likely to crack or make the varnish cloudy.
The success of varnishing de-pends in no small degree on good brushes being used. It is impossible to do good work with a bad brush. Even an expensive brush may prove a failure. So if, after a fair amount of use, a brush is found to be entirely free from loose hairs – which ruin a surface if left adhering – and to produce clean work, it should be treasured and treated with great respect.
When not in use it should be suspended in an airtight vessel with its tip only just touching a mixture of alcohol and turpentine. Some authorities recommend that it should be hung clear, and depend on the vapours for keeping it soft. A simple method of suspension is to pass the handle through a hole in the centre of the cork area as stopper.
The worst thing that can happen to the brush is to be left standing on its bristles, which turn up under the weight and, if left bent long, will refuse to straighten again.
Cold, damp, and dust are foes to varnishing. The first two make varnish whiten, a defect which can be remedied only by removing the varnish and beginning again.
Dust sticks to wet varnish and may ruin the appearance of the work. Varnishing should therefore not be done in a low temperature or dirty surroundings. A room suitable for the work would have a temperature of at least GO degrees, and have been carefully dusted beforehand, wet tea-leaves or sawdust being used on the floor. Windows and doors would be kept shut, to prevent draughts.
In applying varnish, one must not try to cover the surface too quickly. A little at a time, and frequent recharging of the brush are good rules.
If the varnish is applied too thinly, it will not dry with a good gloss; and if too thickly, it will run into ridges. Like paint, varnish should be stroked off along the grain; but it will tend to lather if brushed too much. The air bubbles form, break, and leave tiny but unsightly craters in the dried varnish.
A coat should be given at least a week to dry in before another is laid over it. It is essential that it should have lost all stickiness to the touch.
The varnish, when ready, should be rubbed with very fine pumice powder applied on a wetted felt pad, till the surface is quite smooth, wiped quite clean with a sponge, and allowed to dry for a day. The rubbing down destroys the gloss, but the next coat will restore it. Subsequent coats, except the last, are treated in the same manner. For ordinary work, two coats are sufficient.