Having completed the pattern and its core box, the surface of the wood must be covered with some material which will render it hard, smooth, and impervious to the moisture in the sand, and at the same time make it easier to be withdrawn from the mold. Pure grain-alcohol shellac varnish is the best for this purpose. All cheap substitutes, such as wood-alcohol shellac, or copal varnishes should be avoided; they become flaky and scale off, and do not stand the exposure and moisture. Pattern makers generally make their own shellac varnish, buying only the best quality of shellac gum, and using 95 per cent proof alcohol. The proportions are 3 pounds of gum to 1 gallon of alcohol. The gum is put in a wide-mouthed bottle, or earthen jar, and the alcohol poured over it; and, if well stirred three or four times during the day, this will - if the alcohol is of the best - give a smooth clear orange-colored varnish, ready for use.
A good grade of white grain-alcohol shellac may be made from bleached gum, or can be bought from the dealers, but it dries more slowly and does not produce so hard a surface as the orange shellac. Orange or white shellac varnish should never be kept in a metallic can or cup, as the oxidation of the metal will discolor the varnish. As the alcohol in shellac varnish evaporates very rapidly, the brush should be kept in a vessel which is closed and air tight. A short bottle having a mouth wide enough to admit the brush is best for this purpose. A 1-inch flat double-thickness fitch hairbrush is good for general work. Do not use a cork, but turn a wooden cap for the bottle, such as is shown in Fig. 149, and of which the shoulder at a may be 3/16 inch to 1/4 inch long, but must be at least 1/4 inch less in diameter than the inside of the mouth of the bottle. Otherwise the shellac will cement it to the glass so that it cannot be removed. Its only object is to keep the cap nearly central on the bottle. The handle of the brush must be tightly fitted into a hole through the center of the cap and fastened with a screw or brad, allowing the brush to reach within 1/2 inch of the bottom of the bottle. Keep the bottle one-third to one-half full of shellac, and use the brush with the cap on the handle. The shellac will make a tight joint between the bottle and the cap, and, if the proper amount of shellac is kept in the bottle, the brush will always remain soft.
For small patterns, such as the bushing described, the small quantity of shellac needed can be used directly from the bottle. For large work, however, an earthenware cup or mug should be used, but the shellac left over should always be returned to the vessel in which it is kept.
Having given a perfectly smooth surface to the pattern and core box by the use of very fine sandpaper - No. 0 - apply the first coat of shellac. This first coat will raise the grain and roughen the surface of the wood, which, after the shellac is perfectly dry, must be sandpapered a second time until smooth. Now apply a second coat. Should there still be roughness, a second sandpapering will be necessary. At least three coats of shellac should be used. If there is much end wood exposed on any of the surfaces of the pattern, a fourth coat may be necessary on these parts.
As regards the color in which patterns are finished, there are different rules in different shops. The general rule, however, is to leave all patterns for brass or bronze in the natural color of the wood, and to shellac the core prints red. If the pattern is intended for molding cast iron, the body of the pattern is made black and the core prints red. The parts of the core box in which the core is to be formed are also colored red and the outside of the core box black. The black color is produced by mixing lampblack with the shellac varnish, and the red color by mixing vermilion - Chinese is the best - with the shellac. The vermilion is heavy and will settle, hence it must be stirred or well shaken before using. The best method is to first use two coats of the natural colored shellac orange or white - on all surfaces of the pattern, core prints, and core box, then apply the black or red for the last coat only.
As the pattern already described is for a brass bushing, the body should be left the natural color of the pine, and the core prints on the pattern and the inside of the core box colored red. The outside of the core box may be left the natural color or made black, as preferred. The outside of the core box, having no part in the formation of the core, is not necessarily so well and smoothly finished as the inside.
All nail holes or any defects in the wood should be filled with beeswax applied with the warm blade of a knife, or narrow chisel, warmed by holding in hot water. The beeswax should always be used after the first coat of shellac has been applied, as it will then hold better. The sandpapering of the pattern, after the first coat, will smooth the wax and bring it even with the surface of the wood. The time required for a coat of shellac to dry is from 8 to 12 hours, depending upon how heavily it may have been applied, even though to the touch the surface may seem dry in 1 or 2 hours. If a hard durable surface is required on the pattern, 12 or better, 24 hours must be given between each coat. The roughness will then sandpaper off as a dry powder without gumming the sandpaper, and leave a hard smooth surface for the succeeding coat of shellac.