The Filling of Hard and Soft Woods

BY A. Kelly (1911)

The following woods are called “open-grained” woods and require a paste filler to make a good foundation for a varnish finish: Ash, beech, butternut, baywood, black walnut, chestnut, elm, mahogany, oak, and rosewood.

The following woods are called “close-grained” woods, or softwoods, and should be treated with a liquid filler to fit them for varnish finish: Basswood, cedar, California redwood, gumwood, Oregon pine, poplar, spruce, tamarack, white pine, Washington fir, whitewood, and yellow pine.

The following woods are called “close-grained hardwoods,” and are sometimes filled with paste filler, but this is not generally done, it not being absolutely necessary: Birch, cherry, Circassian walnut, and maple. Fill with liquid filler.

The filler must be coloured to match the wood; it is best to make it rather darker than the wood. It is important to get the right colour for a filler, so that it will be as near the colour of the wood as possible, only a trifle deeper in shade. Again, the colour of the finish may be determined by the filler; that is, the filler will be stain and filler both. Some of the finest colour effects with woods are obtained in this manner.

The purpose of paste filling is to make a solid surface for the varnish coats. The paste enters and seals the pores of the wood and all the open parts. This can only be done on wood having an open grain. But while the cellular structure is thus filled, the fibre is left more or less unfilled, and hence it is customary in many cases to apply a coating of liquid over the paste filling, when dry and rubbed down.

For this purpose, we may use shellac varnish, or a light-bodied liquid filler.

Liquid fillers are used where the wood is not open enough to take in a paste. Its purpose is to saturate the fibre of the wood and thus prevent it from taking up the liquid of the varnish coats, thus robbing the varnish of its oil and turpentine, causing it to be too brittle. Shellac varnish is a liquid filler and is often so used.

Application of Fillers

Liquid filler may be made from paste filler by the addition of the proper thinners. Usually liquid thinner is simply a cheap varnish, with the addition of cornstarch, or clay, or another suitable base. Before the advent of commercial liquid fillers, the surfacing or filling of close-grained woods was done with varnish, applied in several successive coats, each coat being rubbed down and into the pores of the wood by means of a piece of soft white pine, made chisel-shape, and upon this foundation the varnish finish was laid.

Various woods and their adaptability to the different fillers, substances in use, formulas for fillers

The use of a liquid containing some pigment or starch makes it possible to filler surface the wood with one coat. This may be sandpapered down, and a coat or two of varnish will give a finish. We call them surfacers because these liquid fillers are not rubbed into the wood, but laid on the surface, the same as varnish.

The filler or surfacer simply saves us the costlier varnish.

Shellac and Other Substances

Shellac is preferred where cost is not taken into account, because it sandpapers easier than varnish filler, but it is less desirable under varnish than even cheap varnish, because of its hard, inelastic nature, causing cracking of the varnish placed on it, particularly when the shellac is placed between two coats of varnish.

Liquid filler does better over paste filling than varnish, as it seals the open pores better. Liquid filler should be applied much the same as varnish, flowing it on even and smooth. It is best not to colour liquid filler if it is made with silica, because the silica, owing to its weight, will sink and allow the colour to float, giving the surface a painted effect.

Such substances as terra alba, talc, whiting, corn starch and barytes have the fault of whitening or fading out in the wood, a serious defect where any colour is used.

Carbonate of magnesia is very good for holding up the filler, and the same may be said of a few others of this class, but all in all nothing equals finely pulverized silica, whether for paste or liquid filling.

Starch makes a transparent filling, but it is impossible to make a dry starch and varnish filler that will keep long before using. Cooked starch makes the transparent filling, but raw starch will show white in the pores, perhaps worse even than whiting, which also is bad. Even silica is not free from the fault, but is less objectionable than any other mineral filler. Silica does not absorb the liquids of the filler, and, being thus non-absorbent, it is not affected by moisture as is corn starch. It unites mechanically with the fluids of the filler, fills the pores of the wood well, and adheres to the surface perfectly so that finishing over it is easily accomplished. It has been well said by an expert finisher that “a good finish cannot be obtained when starch, earth, and similar substances are used in the filling. “Starch is soft and easily applied, and work can be rushed by using it, and that is the most we can say for it as a filler.

Starch will not hold up the varnish, nor will the application of three or four coats help matters much. Silica can be pushed into the grain of the wood, making a solid foundation on which two coats of varnish will give a splendid finish. Silver-white and pulverized silica look much alike, having much the same atomic or molecular formation. Silver-white is a white siliceous earth found in Indiana. It is much used for making fillers.

One other fault of silica, and it is not a very serious one, consists in the fact that it will settle or not hold up in solutions. Also, it dries out rapidly, but this may be modified by adding a little oil to ‘ it, and in some cases the thinning may be done with oil alone, of which I shall speak presently. However, the fact of its setting so quickly is an evidence that it will be durable.

It should be said here that where large quantities of filler are used, as in the finishing room of a large furniture concern, the barrel of filler should be kept covered, to prevent the evaporation of the liquids, and to keep out dirt and all foreign substances. No one would think of leaving a barrel of varnish with the head out and uncovered, yet filler is composed largely of the same volatile liquids and will oxidize and become hard in like manner.

Referring again to corn starch filler, when it is applied it seems to fill perfectly because it is very absorbent of the liquids and seems to fill the pores of the wood perfectly. In a measure this is true, but in course of time, in the process of drying and hardening, it shrinks and a close examination of the filled wood with a microscope, or even with the naked eye, will disclose a surface full of unfilled pores and this may still be seen after the varnishing has been done. Furthermore, the filler will require much more time for hardening than is ever given it, and the result is seen in the chilling and cracking of the varnish.

Hardwood filler should set in from IS to 20 minutes, and to do this it should not contain an excess of oil, which would retard the drying. Thin, it with turpentine.

Filler Formulae — Liquid Fillers

Shellac varnish is a very satisfactory liquid filler or surfacer, in that it dries quickly and can be sandpapered easily. But it is usually too costly for general practice, besides which it is thought to act more or less badly under oil varnish. When used for surfacing close-grained woods, it should be applied thin. Two coats are better than one. It should be sandpapered down well.

 Imitation shellac may be made. A finisher says he makes one that is not only cheaper than shellac but is better in other ways. He takes equal parts of raw oil.

 Turpentine, brown japan and rubbing varnish, to which he adds enough corn-starch to thicken the mixture, making it rather heavier than ordinary paint, or so it can be applied with a brush. After it has been on the wood long enough to set, he rubs it off with a coarse cloth, rubbing the stuff into the wood at the same time. He applies two coats.

Here is another formula: Take four pounds of either finely pulverized and floated silica or China clay, the former preferred, and stir it into one quart of Japan driers, and beat the mass until perfect admixture takes place. Then add, while stirring the mass, six quarts of the best light hard-oil finish, or other equally good varnish, after which let the mixture stand an hour or so; then strain through a fine sieve. When desired for application, thin up to the proper consistency with turpentine, making it quite thin for liquid filling. It may be used also as a paste filler without thinning.

Oil may be used in place of varnish fora liquid filler for some purposes. Many of the best yachts and steamships have all exposed woodwork filled with an oil-thinned filling, over which is applied a number of coats of elastic varnish, like spar or carriage finishing varnish, with ample time for each coat to dry, and each coat is sandpapered. The process involves time and expense, but it gives a very durable finish when exposed to the weather.

Kaolin, Silica and Other Fillers

Kaolin filler may be made thus: Mix together a gallon of pale body hard-drying carriage varnish, one pint of turpentine, and one pint of pale-drying Japan. Take two and one-half pounds of kaolin and add enough of the mixed liquids to form a paste, which run through closely set hand paint mill, grinding it once, then add the rest of the thinners by brisk stirring, until perfect admixtures secured. Then the filler is fit for use, though it may be further thinned or made stiffer as desired.

Silica paste filler may be thinned down with varnish and turpentine to form a liquid filler. To four pounds of the paste filler add a gallon of coach varnish which may then be thinned with turpentine to a liquid filler consistency.

Liquid filler should be given at least24 hours to dry; 48 hours is better still.

Silver-white filler may be made with equal parts of raw oil, gold size japans and turpentine, with silver-white enough to form a paste, which must be worked smooth. Then it may be thinned with turpentine to the proper consistency.

White liquid filler is made after various formulas, and the following one is as good as any: In a gallon of raw linseed oil put two pounds of pale rosin, powdered, and place on the fire, stirring the mass until the rosin is melted. Take from the fire and add a pint of white japan and two quarts of turpentine; stir all together, and when the mass is cold, add eight ounces of cornstarch. After mixing the starch into the liquid, make it very thin with turpentine, and pass it through a paint mill or strainer.

Some woods require a transparent liquid filler, but such a filler should be made to match the wood in colour, which is of course very light. Mix together eight ounces of cornstarch, eight ounces of finest pumice stone powder and a quarter-gill of white shellac varnish and a quarter-pint of boiled oil. Mix thoroughly together, and thin for use.

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