Wood finishing refers to the process of refining or protecting a wooden surface, especially in the production of furniture where typically it represents between 5 and 30% of manufacturing costs.
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 other wood properties, for example tonal qualities of musical instruments and 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 piece looks attractive, performs well in service and meets safety and environmental requirements. Planning for finishing begins with the design of furniture. Care should be taken to ensure that edges of furniture are rounded so they can be adequately coated and are able to resist wear and cracking. Careful attention should also be given to the design and strength of wooden joints to ensure they do not open-up in service and crack the overlying finish. 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 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. For example, woods that show great variation in colour between sapwood and heartwood or within heartwood may require a preliminary staining step to reduce colour variation. 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 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 preferentially absorb pigmented stain, and advantage can be taken of this to highlight the wood's grain. Some tropical woods, such as rosewood (Dalbergia nigra), cocobolo (Dalbergia retusa) and African padauk (Pterocarpus soyauxii), contain extractives such as quinones, which retard the curing of unsaturated polyester and UV-cured acrylate coatings, and so other finishing systems should be used with these species.
Planning for wood finishing also involves being aware of how the finishing process influences the end result. Careful handling of the wood is needed to avoid dents, scratches and soiling with dirt. Wood should be marked for cutting using pencil rather than ink; however, avoid hard or soft pencil. HB is recommend for face work and 2H for joint work. Care should be taken to avoid squeeze-out of glue from joints because the glue will reduce absorption of stain and finish. Any excess glue should be carefully removed to avoid further damage to the wood.
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 (6 to 8%) that is similar to that of the intended end use of the furniture.
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.