Sanding is carried out before finishing to remove defects from the wood surface that will affect the appearance and performance of finishes that are subsequently applied to the wood.....These defects include cutter marks and burns, scratches and indentations, small glue spots and raised grain. Sanding should not be used to eliminate larger defects such as gouges, and various forms of discoloration. Other techniques are used to remove these defects.
The key to preparing a defect free surface is to develop a sanding schedule that will quickly eliminate defects and leave the surface smooth enough so that tiny scratches produced by sanding cannot be seen when the wood is finished. A sanding schedule usually begins with sandpaper that is coarse enough to remove larger defects (typically 80 or 100 grit, but sometimes higher if the surface is already quite smooth), and progresses through a series of sandpaper grades that gradually remove the sanding scratches created by the previous sanding steps.
A typical sanding schedule prior to wood finishing might involve sanding wood along the grain with the following grades of sandpaper, 80, 100, 120, 150 and finishing with 180 and sometimes 220 grit.The precise sanding schedule is a matter of trial and error because the appearance of a sanded surface depends on the wood you are sanding and the finish that will subsequently be applied to the wood. According to Nagyszalanczy, coarse grained woods with large pores such as oak hide sanding scratches better than fine grained wood and hence with such species it may be possible to use 180 or even 150 grit sandpaper as the final step in the sanding schedule.
Conversely, sanding scratches are more easily seen in finer grained, harder woods and also end-grain, and hence, they require finer sandpaper (220 grit) during the final sanding stage.The sandpaper selected for the final sanding stage affects the color of stained wood, and therefore when staining is part of finishing avoid sanding the wood to a very smooth finish.On the other hand, according to Nagyszalanczy if you are using an oil-based finish, it is desirable to sand the wood using higher grit sandpaper (400 grit) because oil tends to highlight sanding scratches.
Sanding is very good at removing defects at wood surfaces, but it creates a surface that contains minute scratches in the form of microscopic valleys and ridges, and also slivers of wood cell wall material that are attached to the underlying wood.These sanding ridges and slivers of wood swell and spring-up, respectively, when sanded wood is finished with water-based finishes, creating a rough fuzzy surface. This defect is known as grain raising. It can be eliminated by wetting the surface with water, leaving the wood to dry and then lightly sanding the wood to remove the ‘raised grain’.
Removing larger defects
Larger defects that interfere with wood finishing include dent, gouges, splits and glue spots and smears. These defects should also be removed before finishing, otherwise they will affect the quality of the finished furniture or object. However, it is difficult to completely eliminate large defects from wood surfaces.
Removing dents from wood surfaces is quite straightforward as pointed out by Flexner. Add a few droplets of demineralized water to the dent and let it soak in. Then put a clean cloth over the dent and place the tip of a hot iron on the cloth that lies immediately above the dent, taking great care not to burn the wood. The transfer of heat from the iron to the wood will cause compressed fibers in the dent to recover their original dimensions. As a result the dent will diminish in size or even disappear completely, although removal of large dents may require a number of wetting and heating cycles. The wood in the recovered dent should then be dried and sanded smooth to match the surrounding wood.
Gouges and holes in wood are more difficult to repair than dents because wood fibers have been cut, torn and removed from the wood. Larger gouges and splits are best repaired by patching the void with a piece of wood that matches the color and grain orientation of the wood under repair. Patching wood requires skill, but when done properly it is possible to create a repair that is very difficult to see. An alternative to patching is filling (sometimes known as stopping).
Numerous colored fillers (putties and waxes) are produced commercially and are colored to match different wood species. Successful filling of voids in wood requires the filler to precisely match the color and grain pattern of the wood around the void, which is difficult to achieve in practice. Furthermore, filled voids do not behave like wood during subsequent finishing steps, and they age differently to wood. Hence, repairs to wood using fillers may noticeable. Therefore filling is best used with opaque finishes rather than semitransparent finishes, which allow the grain of the wood to be seen.
Glue smears and droplets are sometimes present around the joints of furniture. They can be removed using a combination of scraping, scrubbing and sanding. These approaches remove surface glue, but not the glue beneath the wood surface. Sub-surface glue will reduce the absorption of stain by wood, and may alter the scratch pattern created by sanding.
Both these effects will influence the way in which the wood colors when stains are used to finish the wood. To overcome this problem it may be necessary to locally stain and touch-up areas previously covered by glue to ensure that the finish on such areas matches that of the surrounding wood.
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