Types of Wood Fillers

¶ Wood fillers are needed for the purpose of filling the open cells or tiny crevices in open-grain woods such as oak, ash, chestnut, butternut, elm, mahogany, walnut, etc. All woods are composed of fibres and cells. While growing these cells are filled with water, sap or resin. When the wood is seasoned and dried the cells are filled with air. The surface cells are little holes or pores. In the open-grain woods listed above they are fairly large. In close-grain woods like maple, birch, bass, beech, cherry, cypress, fir, pine, gum, poplar, redwood, spruce, sycamore and holly, the cells are very small, the fibre being closely woven together.

¶ Fillers are of two kinds, paste and liquid. Paste fillers usually are semi-transparent but sometimes are opaque. Liquid fillers are transparent. The paste fillers are used on open-grain wood, while the liquid fillers are used usually on close-grain wood.

¶ The purpose served by fillers is that of leveling, by filling, the cells of the wood. They are used to build tip the tiny depressions in the wood, in other words, and thus prevent the varnish finish from sinking in and from giving a pitted or rippled effect. This effect is apt to remain even though several coats of varnish are put on, unless the under coats of varnish are rubbed down a great deal, when they act as a filler. It is cheaper and quicker to fill the surface before varnishing. The filler, however, should be as transparent as possible in order to preserve the natural color beauty of the wood. Although in some cases, like the two-tone finishes, the filler is purposely made opaque to gain a special effect.

¶ The close-grain woods are often finished without the use of any filler. And also the open-grain woods are finished without any filler to gain the correct effect for such finishes as weathered oak, fumed oak, Flemish oak, Flanders oak, antique oak, ebony, Jacobean.

¶ In the best classes of work the filler is put on after the stain, but in certain special cases and for cheaper work where it is desirable to save the labor of putting on a separate stain coat, the filler and stain are put on in one coat. The stain does not penetrate so deeply when combined with the filler coat but satisfactory results can be secured, especially if about 20% of the thinner liquid for the filler is composed of benzole, 90 degree, or solvent naphtha, 160 degree. When oil stains are to be used some finishers prefer to put the filler on first because filler containing oil put on over an oil stain is apt to lift the stain, mix it with the filler and produce a muddy, cloudy finish. The best practice, however, is to put on even the oil stain first, let dry and then seal it up with a very thin coat of shellac, about a 2-pound cut before the filler is spread on the wood. A thick coat of shellac which is not cut. down by considerable rubbing makes a poor foundation for a finish. When it is rubbed enough to cut off all shellac except that which is lodged in the pores of the wood it makes an excellent filler.

¶ One pound of paste filler will fill about forty square feet of surface. One gallon of liquid filler will fill from 200 to 400 square feet, depending upon its composition.

¶ The importance of proper filling of wood is indicated by the fact that the expert furniture finishers inspect a filled surface after it is dry with a magnifying glass.

¶ If he finds numerous pinholes in the filled surface, indicating that the filler bridged-over the open pores rather than entered and filled them, he brushes on a second coat of filler of thinner consistency, lets it dry hard, sandpapers down to the bare wood, then, removing all except what is lodged in the pores.