Porous woods, such as oak and ash, take a smoother and more durable finish if a good paste filler is rubbed into them before the varnish, wax, or shellac is applied. Maple, pine, and other nonporous woods do not need such treatment and in fact will not absorb some kinds of fillers.
The best paste fillers are made of silex (silica), linseed oil, turpentine, japan, and coloring matter to match the wood. Cornstarch and whiting are also used as the base of paste fillers, but are less transparent than silex and can not be worked into the pores of the wood so thoroughly. They are generally used in homemade fillers, however, for silex is difficult to obtain in the retail trade. Oil has a tendency to darken wood, so it is sometimes omitted from the filler if a very light finish is desired.
A filler should be about the consistency of varnish when applied. If too thick, it can be thinned with turpentine for use on natural-colored woods, or with boiled linseed oil on stained woods. After the floor has been dusted, the filler is generously applied lengthwise of the grain with a clean stiff brush. This coating is allowed to set for 15 or 20 minutes, or until it turns gray, and is then rubbed in with cotton waste or burlap crosswise, not lengthwise, of the grain. A coarser material will drag the filler out of the pores instead of forcing it in. Several days later the floor is rubbed smooth with No.0 sandpaper slightly dampened on the back. Ordinary oak will take up about 5 pounds of filler to 250 square feet of floor. If a very high polish is desired, a second coat of filler containing less oil and more turpentine may be applied and rubbed down as in the first case.
Liquid fillers are sometimes used on close-grained woods to fill up the pores and prevent the absorption of the more expensive varnish. A pure shellac varnish made by dissolving gum shellac in alcohol is recommended by some authorities for this purpose. The ready-mixed liquid fillers, which are brushed on and permitted to remain on the surface without being rubbed off, are in many cases little better than cheap varnishes.