Wood finishing

Finishing of wood requires careful planning to ensure that the finished piece looks attractive, performs well in service and meets safety and environmental requirements. Planning for finishing begins with the design of furniture. Care should also be taken to eliminate recesses in furniture, which are difficult to finish with some systems, especially UV-cured finishes. Planning for wood finishing also involves thinking about the properties of the wood being finished, as these can greatly affect the appearance and performance of finishes, and also the type of finishing system that will give the wood the desired characteristics. Alternatively, the wood can be bleached to remove the natural colour of the wood and then stained to the desired colour. Woods that are coarse textured such as oaks and other ring-porous hardwoods may need to be filled before they are finished to ensure the coating can bridge the pores and resist cracking. The pores in ring-porous woods more readily absorb pigmented stain, and advantage can be taken of this to highlight the wood's grain. Planning for wood finishing also involves being aware of how the finishing process influences the result. Careful handling of the wood is needed to avoid dents, scratches and soiling with dirt. Changes in wood moisture content can result in swelling and shrinkage of wood which can stress and crack coatings. Both problems can be avoided by storing wood indoors in an environment where it can equilibriate to a recommended moisture content (6 to 8%) that is similar to that of the intended end use of the furniture. local environmental regulations governing the use of finishes, and recycling of finished wood at the end of its life. == Sanding ==

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. 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. 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. Numerous coloured fillers (putties and waxes) are produced commercially and are coloured to match different wood species. Successful filling of voids in wood requires the filler to precisely match the colour 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 colours 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. == Bleaching and removal of stains ==

Wood surfaces are occasionally affected by various organic and inorganic stains. Sometimes such stains enhance the colour and appearance of wood. and there is no reason to remove the stain from the wood prior to finishing. The same applies to spalted wood whose attractive appearance is again caused by fungi. On the other hand, some fungal stains and those caused by the reaction of iron with wood can disfigure wood. Bleaches are also occasionally used to reduce the difference in colour between lighter sapwood and heartwood and also colour variation within heartwood. Such bleaching make it easier to achieve a uniformly coloured wood when the wood is subsequently coloured with pigmented stains and dyes (see below). Furthermore, the natural colours of wood fade when wood is exposed to sunlight, and more permanent colours can be created by bleaching wood to remove its natural colour and then re-colouring the wood using artificial, light-fast, stains. The bleaches used to remove unwanted fungal stains from wood include two-part peroxide bleach and solutions of sodium hypochlorite. The former is particularly effective at removing the natural colour of wood before it is recoloured with pigmented stains or dyes. Oxalic acid is particularly effective at removing iron stains from wood. == Colouring wood ==

Wood can be stained to change its colour or left unstained before application of lacquer, or other types of top-coats. Staining should enhance the appearance of wood by reducing colour variation between and within sapwood and heartwood. It also provides a way of giving bland looking woods such as poplar the appearance of prized furniture woods such as ebony, mahogany or walnut. Wood can be stained using dyes or pigmented finishes. These finishes are available in a wide variety of colours, many of which are not part of the natural colour palette of wood, for example, blues and greens. Pigmented stains tend to highlight the grain (and also sanding scratches), whereas dyes do not have this effect and are more transparent. Wood can also be coloured by exposing it to chemicals that react with the wood to form coloured compounds. Chemical staining of wood is rarely carried out because it is easier to colour wood using dye or pigmented stain, however, ammonia fuming is a chemical staining method that is still occasionally used to darken woods such as oak that contain a lot of tannins. Staining of wood is difficult to control because some parts of the wood absorb more stain than others, which leads to problems such as blotchiness and streaking. For this reason, as pointed out by Flexner, many people prefer to omit the staining step when finishing wood. ==Basic wood finishing procedure==

Wood finishing starts with sanding either by hand, typically using a sanding block or power sander, scraping, or planing. Imperfections or nail holes on the surface may be filled using wood putty or pores may be filled using wood filler. Often, the wood's color is changed by staining, bleaching, or any of a number of other techniques. Once the wood surface is prepared and stained, the finish is applied. It usually consists of several coats of wax, shellac, drying oil, lacquer, varnish, or paint, and each coat is typically followed by sanding. Finally, the surface may be polished or buffed using steel wool, pumice, rotten stone or other materials, depending on the shine desired. Often, a final coat of wax is applied over the finish to add a degree of protection. French polishing is a finishing method of applying many thin coats of shellac using a rubbing pad, yielding a very fine glossy finish. Ammonia fuming is a traditional process for darkening and enriching the color of white oak. Ammonia fumes react with the natural tannins in the wood and cause it to change colours. The resulting product is known as "fumed oak". ==Comparison of different clear finishes==

Clear finishes are intended to make wood look good and meet the demands to be placed on the finish. Choosing a clear finish for wood involves trade-offs between appearance, protection, durability, safety, requirements for cleaning, and ease of application. The following table compares the characteristics of different clear finishes. 'Rubbing qualities' indicates the ease with which a finish can be manipulated to deliver the finish desired. Shellac should be considered in two different ways. It is used thinned with denatured alcohol as a finish and as a way to manipulate the wood's ability to absorb other finishes. The alcohol evaporates almost immediately to yield a finish that will attach to virtually any surface, even glass, and virtually any other finish can be used over it. File:Piece04-NoFinish.jpg|No finish File:Piece01-LinseedOil.jpg|Linseed oil File:Piece10-TungOil.jpg|Tung oil File:Piece11-TungOil-LinseedOil.jpg|Tung oil and linseed oil File:Piece05-BeeWax-Molten.jpg|Molten bee wax File:Piece03-CarnaubaBeeWax.jpg|2:1 ratio of beeswax and carnauba wax File:Piece02-ShellacOrange.jpg|Shellac File:Piece06-Shellac-LinseedOil.jpg|Shellac and linseed oil File:Piece08-PUR-Alkyd-Varnish.jpg|Alkyd varnish File:Piece09-Spar-Varnish.jpg|Spar or yacht varnish File:Piece12-PU-Acrylic-Varnish.jpg|Acrylic varnish File:Piece07-Acrylic-Paint.jpg|Acrylic paint 1 accentuates visual properties due to differences in wood grain. ==Automated wood finishing methods==

Manufacturers who mass-produce products implement automated flatline finish systems. These systems consist of a series of processing stations that may include sanding, dust removal, staining, sealer and topcoat applications. As the name suggests, the primary part shapes are flat. Liquid wood finishes are applied via automated spray guns in an enclosed environment or spray cabin. The material then can enter an oven or be sanded again depending on the manufacturer's setup. The material can also be recycled through the line to apply another coat of finish or continue in a system that adds successive coats depending on the layout of the production line. The systems typically used one of two approaches to production. Hangline approach In the hangline approach, wood items being finished are hung by carriers or hangers that are attached to a conveyor system that moves the items overhead or above the floor space. The conveyor itself can be ceiling mounted, wall mounted or supported by floor mounts. A simple overhead conveyor system can be designed to move wood products through several wood finishing processes in a continuous loop. The hangline approach to automated wood finishing also allows the option of moving items up to warmer air at the ceiling level to speed up drying process. Towline approach The towline approach to automating wood finishing uses mobile carts that are propelled by conveyors mounted in or on the floor. This approach is useful for moving large, awkward shaped wood products that are difficult or impossible to lift or hang overhead, such as four-legged wood furniture. The mobile carts used in the towline approach can be designed with top platens that rotate either manually or automatically. The rotating top platens allow the operator to have easy access to all sides of the wood item throughout the various wood finishing processes such as sanding, painting and sealing. ==See also==