Wood finishing refers to the process of refining or protecting a wooden surface.
Finishing is the final step of the manufacturing process that gives wood surfaces desirable characteristics, including enhanced appearance and increased resistance to moisture and other environmental agents. Finishing can also make wood easier to clean and keep it sanitized, sealing pores that can be breeding grounds for bacteria. Finishing can also influence the hardness of flooring. In addition, finishing provides a way of giving low-value woods the appearance of ones that are expensive and difficult to obtain.
Finishing of wood requires careful planning to ensure that the finished floor looks attractive, performs well in service and meets safety and environmental requirements.
Planning for wood finishing also involves thinking about the properties of the wood that you are going to finish, as these can greatly affect the appearance and performance of finishes, and also the type of finishing system that will give the wood the characteristics you are seeking. Wood’s moisture content affects staining of wood 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 stored wood indoors in an environment where it can equilibrate to a recommended moisture content.
Finally, consideration needs to be given to whether the finished wood will come into contact with food, in which case a food-safe finish should be used, local environmental regulations governing the use of finishes, and recycling of finished wood at the end of its life.
Wood surfaces are occasionally affected by various organic and inorganic stains. Sometimes such stains enhance the colour and appearance of wood. For example, oak wood affected by the beef-steak fungus has a deep rich, attractive, brown colour 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. These stains can be removed from wood using bleach. Bleaches are also occasionally used to reduce the difference in colour between lighter sapwood and heartwood and also colour variation within heartwood. Such bleaching makes it easier to achieve a uniformly coloured wood when the wood is subsequently coloured with pigmented stains and dyes. 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.
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. 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.