CLEANING, SURFACING, AND FILLING
First Steps in Finishing.—The first step in finishing woodwork is to clean it up and prepare it for the filler. Too great care cannot be taken in examining all surfaces for slight oversights that have occurred in the joinery work or for blemishes that have been acquired in any part of the process of construction. Light planing, chiselling, or scraping, and perhaps a little sand-papering, always with the grain, may be needed to remove these faults and blemishes. The most serious of these generally result from gluing. All surplus glue should be carefully removed, even to the film that soaks into the surface of the wood. The slightest spot of glue remaining will keep the filler out of the wood and show through any kind of finish.
Importance of Filling.—As soon as the furniture is thoroughly cleaned, if it is to be finished in the natural color, the grain of the wood must be filled with a light, transparent, semi-liquid substance, known as the filler, which, after standing from five to seven minutes, should be thoroughly rubbed off with a piece of coarse cloth or a handful of shavings. This process, as its name indicates, fills the pores of the wood and protects them against the absorption of moisture and the consequent swelling. But this is by no means its chief purpose. Strictly speaking, filling is the ground-work of all subsequent finishing processes and, for that reason, it may be said to be the most important operation connected with wood finishing. If improperly done, no amount of good work with the varnish brush will remove the fault. In fact, good work is impossible after a bad beginning. Good varnishing or good finishing of any kind requires that, before the final finishing processes begin, the surface should be made perfectly level and smooth, free from all unevenness or roughness or minute openings of any kind that might allow the varnish or other finishing material to penetrate the wood. If the filling has not been so thorough that no part of the subsequent finishing material can be absorbed by the wood, a rough surface will be sure to follow. This may, of course, be rubbed down and re-finished, but never with that perfection of result which is assured by laying a good foundation in correct filling. It may be set down as a fundamental principle of wood finishing that the best result is obtained when the filling or surfacing has been so thoroughly done that the thinnest of finishing coats, few in number, are required to produce the desired result.
Liquid Fillers or Surfaces.—There are many kinds of fillers in common use, but they may all be considered as belonging to one or the other of two classes. They are either liquid fillers or paste fillers. Woods differ greatly in the coarseness or fineness of their grain. The coarse or open-grained woods require a filler with body enough to close up the pores of the wood and give that perfectly even surface necessary as a foundation for good finishing. The close-grained woods, on the other hand, do not present the same filling problem. They could not absorb a paste filler. In fact, it is hardly too much to say that the fine, close-grained woods, like maple, gum wood, and birch, do not need a filler at all; and there are some coarse-grained woods, like southern or hard pine and cypress, the pores of which are naturally filled with gummy or resinous substances and will not absorb an artificial filler. But all such woods do need to be given a finishing surface which will prevent the finishing coats from soaking into the fibre of the wood. This is the office of the so-called liquid fillers, which are very properly called "first coaters" or "surfacers."
How to Make a Liquid Filler.—A standard formula for the preparation of a liquid filler is as follows: Mix four parts by weight of carbonate of soda with six parts of china clay, and grind this mixture in about eight parts of japan, thinning the product with turpentine or benzine to the consistency of linseed oil. Laundry starch may be used in place of china clay, giving a filler which is somewhat easier of application than the clay filler because it does not settle. It lacks in durability, however, especially if it is not well covered. The finest grained woods do not require the addition of any material to the filler to give body. A great variety of liquids may be easily obtained which, without being mixed with anything, will give the necessary surface.
Glue size, water glass, and the cheaper grades of varnish, thinned if necessary with benzine or turpentine, are often used for this purpose. But these are all inferior to the standard surfacers and never should be used on the best grades of woodwork. Shellac is always preferred as a first coater for hard pine, as it keeps the resinous sap in the pores of the wood and preserves the natural colour of the grain. If oil is applied to hard pine without first applying this protecting coat of shellac, the wood blackens with age. Shellac is an excellent first coater for other woods also. It is commonly used in house finishing as the surfacer for the interior trims. But it dries rapidly, and generally with a rough surface. The first coating of shellac, therefore, requires careful sand-papering before the varnish is put on.
Time Needed for a Shellac Coat to Dry.—It may be well to caution the amateur finisher as to the time required for the thorough drying of shellac. While it may be truly said to dry very rapidly, the first drying is necessarily upon the outer surface, forming a hard coating which delays somewhat the drying throughout, so that a shellac filling is not really dry enough to sand-paper a half hour or so after it has been applied, though it may appear to be. It is a good rule to allow at least twenty-four hours for thorough drying; and it may be well to add that all methods of filling, rubbing off, etc., require for the best results more time than energetic workers sometimes allow.
Cost of Surfacing.—A practical question that the amateur finisher will ask is, how can one know in advance how much filler is needed for given pieces of work, and what will be its cost? For a good quality of liquid filler it is safe to say that one pint will cover eight square yards with one coat. The cost varies greatly, depending upon the quality of the filler and whether it is home-made or obtained of a dealer. A good commercial filler or surfacer can be bought for $1.50 per gallon, making the cost per square yard of one-coat surfacing about 21⁄2 cents. A small can costs 15 cents. Good work cannot be done with the cheaper grades of filler. Shellac, which is, all in all, the best of surfacers, can be bought for $1.85 a gallon, which would make the cost of surfacing somewhat under 3 cents a square yard.
How to Make a Paste Filler.—A good paste filler, such as is required for the open-grained woods like oak, ash, baywood, and poplar or white wood, may be made from pulverized and floated silica as a base, thoroughly mixed with raw linseed oil, turpentine, and japan in equal parts, with enough silica added to the liquid mixture to form a good paste of a consistency somewhat thicker than paper-hangers' paste. If the mixture should prove to be a little too thick, it may easily be thinned with turpentine. The final mixing of this filler requires grinding in a hand mill. Unless a considerable quantity of it is needed it is quite as well to purchase a can of the paste filler of a dealer in painters' or finishers' supplies, though commercial fillers are not generally quite so good as a one made on this formula. They often contain wax and whiting and other materials as a base which are not so satisfactory in the long run as the floated silica.
How a Paste Filler is Applied.—Paste fillers should be spread on the surface to be filled very liberally with a wide, stiff brush, allowed to stand from five to seven minutes, and then rubbed off with a piece of burlap or a handful of fine shavings or excelsior. But it must not all be rubbed off or drawn out of the grain. This is an easy mistake for an inexperienced worker to make. The thing to do is to rub it in and at the same time leave the surfaces smooth. The surfaces should then be allowed to dry for about twenty-four hours before receiving the final finishing coats.