In varnishing flat surfaces the varnishes are all applied like paint, with brushes that should be soft, and perfectly clean. For spirit varnishes, camel's hair pencils and brushes are used, the sizes of which vary from one quarter to three-quarters of an inch diameter, according to the size of the work. When the surfaces are very large, flat camel hair brushes are used; but from their comparative thinness, they scarcely contain a sufficient quantity of varnish to preserve the brush uniformly charged in passing over a large surface. Turpentine and oil varnishes require less delicacy, and flat brushes, made of fine soft bristles, are generally used, or sometimes ordinary painting brushes are employed; but they are rather harsh, and, owing to the adhesion of the varnish, the hairs are apt to be loosened, and come out.
The varnishes should all be uniformly applied, in very thin coats, very sparingly upon the edges and angles, where the varnish is liable to accumulate; and a sufficient interval of time should be allowed between every coat for the perfect evaporation of the solvent, whether alchohol, turpentine, or oil. The time required for this depends partly on the kind of varnish employed, and partly on the state of the atmosphere; but, under ordinary circumstances, spirit varnishes generally require from two to three hours between every coat. Turpentine varnishes mostly require six or eight hours, and oil varnishes still longer, - sometimes as much as twenty-four hours. But whatever time may be required, the second layer should never be added until the first is permanently hard; as when one layer is defended from the air by a second, its drying is almost entirely stopped, and it remains soft and adhesive. Every precaution should also be taken to prevent any dust, or loose hairs from the brush, becoming accidentally attached to the varnish; should this occur, they should be immediately removed before the varnish drys, or otherwise they will require to be carefully picked out with the point of a pen-knife, and the surface of the varnish levelled with fine glass-paper, prior to the application of the next coat. In using spirit varnishes it is at all times of the first importance that particular attention should be bestowed upon carrying on the varnishing in a dry atmosphere; as all solutions of resins in alcohol are precipitated by the addition of water, not only as visible moisture, but even as vapour, which is at all times deposited by the atmosphere at a reduced temperature, in the form of invisible dew, and in this state it precipitates the resin in the thin coat of varnish, and gives the surface a milky, opake, or clouded appearance, when the varnish is said to be chilled; but this effect is frequently produced even on a warm and apparently fine summer day, when the atmosphere happens to be more than usually charged with moisture. This is a frequent stumbling block in varnishing, and is only to be obviated, by carrying on the process in a room sufficiently warmed to keep the moisture suspended in the air, until the solvent has entirely evaporated, and left the resin as a thin glassy coat but little altered, in a chemical point of view, from its primary state of fragment, flake, or grain, and entirely unacted upon by water, upon which circumstance the brilliancy and defensive value of the varnish depends.
Not only should the room be sufficiently heated, but all currents of cold air must be avoided, as cold draughts from the interstices of the door or window, if suffered to pass over the recently varnished surface, are quite sufficient to dull the varnish wherever they extend. When the varnish has been chilled, the brilliancy and clearness may frequently be restored by giving the chilled surface another thin coat of varnish, taking care to avoid the causes of the former failure, and immediately holding the varnished surface at a moderate distance from a fire, so as to warm it sufficiently to partially redissolve the chilled coat; but care is necessary to avoid heating the varnish so much as to raise blisters, which would spoil the surface, and no remedy would remain but to rub off the entire coat of varnish with glass-paper, and recommence the process.
The temperature generally preferred for the varnishing room, is about 72° F.; but a few degrees more or less are not very important. The works to be varnished should be kept in the room for a few hours before varnishing, in order that they may acquire the same temperature as the atmosphere, and the surfaces should be smoothed with fine glass-paper, to remove all traces of moisture or grease, and if it should be necessary to stop any minute holes in the wood before varnishing, it should be done with some of the gums, or with wax, or at all events, nothing containing oil or grease should be employed.
An ordinary preserve-jar is frequently used for containing the varnish, and is sufficiently suitable; but it is desirable to have a wire or string fixed across the top, for reducing the quantity taken up by the brush, which is wiped against the wire every time that it is dipped into the varnish. The quantity of varnish poured into the jar should "be sufficient to nearly cover the hairs of the brush, in order to keep it soft. Too small a quantity of varnish is liable to thicken rapidly by evaporation, which should at all times be prevented, as far as possible, by keeping the vessel closely covered when not actually in use. Should the varnish, however, become too thick, it may be readily thinned by the addition of spirits of wine, and for good work it is more desirable to apply an increased number of thin coats than to use the varnish when too thick, as the surface is then almost certain to appear irregular, and full of lines.
In applying spirit varnish, some little tact and expedition are necessary, in order to spread the varnish uniformly over the surface before it becomes too much thickened by evaporation, or it will exhibit a very irregular surface when finished. If the surface does not exceed a few inches square, no material difficulty is experienced, as the whole may be brushed over two or three times before the varnish becomes too thick; but surfaces containing two or three square feet present much greater difficulty, as it is necessary that the varnish should be sufficiently worked with the brush, to exclude all minute air-bubbles, which would spoil the appearance of the work, and can seldom be entirely-removed until just before the varnish is becoming too thick to flow or spread uniformly after the brush has passed over it.