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a pasty substance used in painting and varnishing to produce a smooth surface before the surface coat is applied. In addition to film-forming materials, fillers contain extenders such as chalk, talc, and barite as well as pigments such as zinc white and ocher. Fillers may have a varnish, drying-oil, or natural-adhesive base. Since the content of film-forming materials in a filler is 5–12 times lower than that of pigment or extender, fillers do not provide for a sure adhesion of the covering coats to the surface. For this reason, a filler is usually applied over a layer of primer.
A thick filler is applied with a putty knife or a piece of rubber. A filler thinned with small amounts of solvent may be applied with a pneumatic spray gun. The thickness of a layer of filler may reach 300 μ; when a filler is used to fill holes or to seal a joint or a riveted or welded seam, however, several layers with a total thickness of up to 1 mm may be required. The dried-up layer of filler must be sanded well with an abrasive paper. Varnish-based fillers are used mainly in the machine-building industry; fillers with a natural-adhesive or drying-oil base are used primarily in the construction industry.