The last and finishing process in japanning consists in the laying on and polishing the outer coats of varnish, which are equally necessary whether the plain japan ground be painted on or not. This is generally best done with common seed-lac varnish, except on those occasions where other methods have been shown to be more expedient; and the same reasons which decide as to the propriety of using the different varnishes as regards the colours of the ground, hold equally with those of the painting; for where brightness is a material point, and a tinge of yellow would injure it, seed-lac must give way to the whiter resins; but where hardness and tenacity are essential, it must be adhered to; and where both are necessary, a mixed varnish must be adopted. This mixed varnish should be made of the picked seed-lac, as directed in the case of the white japan grounds. The common seed-lac varnish may be made thus: - Take three ounces of seed-lac, and wash it well in several waters; then dry it and powder it coarsely, put it, with a pint of rectified spirit of wine, into a bottle, so that it be not more than two-thirds full; shake the mixture well together, and place the bottle in a gentle heat till the seed appear to be dissolved, the shaking being in the meantime repeated as often as may be convenient; and then pour off all the clear, and strain the remainder through a coarse cloth.
The varnish thus prepared must be kept for use in a bottle well stopped. The whiter seed-lac varnishes are used in the same manner as the common, except with regard to the substance used in polishing; which, where a pure white, or great clearness of other colours is in question, should be itself white; while the browner sorts of polishing dust, as being cheaper, and doing their business with greater dispatch, may be used in other cases. The pieces of work to be varnished should be placed near the fire, or in a warm room, and made perfectly dry, and then the varnish may be laid on with a flat camel-hair brush made for the purpose: this must be done very rapidly, but with great care; the same.place should not be passed twice over, in laying on one coat, if it can possibly be avoided: the best way of proceeding is to begin in the middle, and pass the brush to one end, then, with another stroke from the middle, pass it to the other end, taking care that, before each stroke, the brush be well supplied with varnish. When one coat is dry another must be laid over it in like manner, and this must be continued at least five or six times.
If, on trial, there be not a sufficient thickness of varnish to bear the polish, without laying bare the painting or ground colour underneath, more must be laid on. When a sufficient number of coats is thus laid on, the work is fit to be polished; which must be done, in common cases, by rubbing it with a piece of cloth, or felt, dipped in tripoli, or pumicestone finely powdered. But towards the end of the rubbing a little oil of any kind should be used with the powder; and when the work appears sufficiently bright and glossy, it should be well rubbed with the oil alone, to clean it from the powder, and give it a still greater lustre. In the case of white grounds, instead of the tripoli, fine putty or whiting should be used, but they should be washed over to prevent the danger of damaging the work from any sand, or other gritty matter, that may happen to be mixed with them. It greatly improves all kinds of japan work to harden the varnish by means of heat, which, in every degree that it can be applied, short of what would burn or calcine the matter, tends to give it a more firm and strong texture.
Where metals form the body, therefore, a very hot stove may be used, and the work may be continued in it a considerable time, especially if the heat be gradually increased; but where wood, or papier mache is in question, heat must be sparingly used.