Paste Wood Fillers
How to mix and use paste wood fillers.
¶ Factory Made Paste Fillers
¶ Much of the filler used today is ground and mixed in the paint factories. It comes in paste form which is quite thick and is ready for use after being thinned with turpentine, mineral spirits or benzine.
¶ The colors commonly listed by paint supply houses are: Transparent or Natural; Walnut; Mahogany, light; Mahogany, dark; Golden Oak; Ebony; Special White.
¶ These paste fillers are sold in cans of the following sizes: 1, 5, 10 and 25 pounds each.
¶ The chief advantages gained by using these fillers are that the quality is good, the fillers adhere firmly to the wood and the colors are standard. You can mix batch after batch of the same color, securing a match. The working qualities are also standardized. Success in filling depends largely upon tempering the filler just right with the thinner liquids. So when the factory-made fillers are always made the same way and you become accustomed to working with a particular brand, time is saved by knowing exactly how the filler will work every time.
¶ Of the factory-made liquid fillers it must be said that some are good and some are not. Those which are made with good varnishes are very good, while those made with the cheap varnishes are not to be depended upon. In this class of goods it is unwise to buy any but the best brands of reputable manufacturers and pay the price of good material.
¶ Painter Mixed Paste Fillers
¶ For a great many years finishers have mixed fillers by various formulas to serve a variety of surfaces. Such fillers are colored to match the stain used, usually but not always, with tinting colors ground in oil or dry colors. Mahogany filler is often colored black or very dark brown. Fillers for silver gray and other grays are white or light gray. For novelty finishes fillers of strongly contrasting colors are used on natural colored woods or upon stain colored woods. The color of the filler is very important when producing any of the period finishes or others of known standard effects. The color of the filler then must be correct or you cannot match the samples with the stain color alone. For this coloring of fillers it is far better to use tinting colors ground in oil than dry colors, because the ground colors are thoroughly mixed and very fine which makes them compact. Compactness is essential, indeed, in good fillers.
¶ The basic materials used for mixing fillers must be ground very fine and ought to be as transparent as possible. The chief material used for high class fillers is silica (silex). It is doubtful if any other material is as good as very fine, water-floated silica. It is white when dry but, being quite transparent, it takes on the color of whatever substance it is mixed with. Silica is a white sand made by crushing quartz rock. It is much like powdered glass and, in fact, glass is also made from silica. Silica is inert chemically, showing no unfavorable reactions when mixed with other pigments and liquids.
¶ Good filler must not shrink or expand, must not attract moisture and should be heavy enough to avoid carrying air cells into the pores of the wood. Silica filler when properly mixed sinks into the pores of the wood and does not expand, shrink, bulge or bleach out in the wood grain. These stable qualities make it an ideal pigment for this purpose.
¶ Some of the other pigments used for mixing fillers are whiting, china clay, cornstarch, barytes, reno raw umber filler and keystone filler. Each is good for special purposes, but it is doubtful if any equals silica for general use.
¶ A commonly used formula for mixing paste filler is:
Mix the above liquids and then add fine silica until you have a very thick paste which is well mixed. Put the paste through a paint mill or mix it thoroughly with a paddle and strain through a wire screen. Thin with benzine or naphtha to thick brushing consistency;it should be just thin enough to pour out of a pot. Next add the tinting colors ground in oil to gain the color wanted. The colors should be broken tip with turpentine or benzine and strained before adding them to the filler. While in use the filler should be stirred every few minutes, because the heavy pigment settles rapidly to the bottom of the pot and you will not have a well-filled job unless the filler is kept to the same consistency all the time it is being used.
¶ When filler is to be made up in advance to keep for some time the above paste filler will separate, the pigment will settle to the bottom. In order to overcome this add cornstarch to the extent of from 10% to 20% of the weight of the silica used. Or add up to 25% of asbestine (silicate of magnesia). Such addition will keep the filler pigment suspended in the oil.
¶ Paste filler mixed after the preceding formula is correct for natural colored finishes if the tinting colors are omitted. The silex or silica is white while dry but the filler when mixed without coloring matter takes on the color of the oil and is correct for natural finishes.
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