WOOD SURFACE FINISHING
Post 472 by Gautam Shah
Woods’ surfaces have some inherent qualitative characteristics. These relate primarily to the species of wood, broadly the Softwoods and Hardwoods. Other features include presence of oils or resins, food transfer cells, pores, nature of grains (straight, entwined, interlocked, curly or mottled), and local variations of grain colours. Timbers in spite of good seasoning practices show changes in the wood surface during various seasons, stresses and over long term conduct. Wood products are created from young timbers (freshly seasoned) as well as reuse of aged timbers. Wood finishes, are applied on fresh timbers, for conservation of existing status, and for rejuvenation or reformation exercises.
Wood finishes are broadly of three classes: 1. Finishing done to timbers, 2. Finishing fashioned after a product or item is formed and 3 Finish processes for repairing or reconditioning an existing product.
1 Finishing done to timbers is accomplished with sizing and shaping operations, and include levelling and straightening of the surface.
2 Finishes fashioned after a product or item is formed, are of two types: Finishing with tools, relating to micro-removal of surface material, by grinding, sandpapering, burnishing, singeing, etc. Finishing, a surface, with applique materials, such as surface sealing, putty application, staining and coating. In some instances it may include covering with film, foil or printing.
3 Finish processes for repairing or reconditioning an existing product may involve full or partial removal of existing applied finishes, re-levelling or straightening of an original timber surface to correct deformation of bending, warping, surface irregularities caused by differential shrinking. The removal of existing applied finish is most difficult as it is a matter of conservation, preservation, correction, all in a very limited scale of intervention.
SOFTWOODS are in dull in colour, light in weight and soft grained, but not always inferior in quality. The sap and heart portions are not very different in colour. Soft woods are easy to finish by planing and sanding. A finished surface exposes the tender portions, the remnants of food transfer areas. And these are likely to shrink and decay over a long period, in spite of seasoning. Small amounts of aliphatic compounds, waxy and resinous substances give a fresh wood, a short lasting, smooth feel and slight sheen. Softwoods due the grain structure and the constituents are difficult to stain. Its surface cannot be well sealed and levelled by chalk or oxide pigments, but a coat of very low viscosity NC Lacquer can seal the face.
Softwood articles are difficult to restore, as the wood grains of aged article show uneven settlement. To correct this, entire applique coating must be removed, and surface re-ground, or heavy surface filling by low opacity minerals is required. Old timber articles show very dry surface, which is prone to chipping along the grain.
HARDWOODS are darker in colour and heavier in weight. In hard woods, the heart portions are fairly distinguishable from the sap portions. On planing and sanding the hardwood surface, intermittent branches of pores, the food transfer areas are clearly visible. The pores are very narrow in width and short in length. The pores on drying tend to shrink in but being fewer and tightly packed by the surrounding fibres, effect of moisture transfer is not very acute as with soft woods. Hardwoods are tough grained and require greater efforts for smoothing, but the finished surface retains its fairness much longer.
Rough finishes are cheaper, take less time to prepare and require simple tools and techniques. Some rough surfaces give better bondage to preservatives and coatings. Rough surfaces are good for moisture movement but are highly vulnerable to insect and bacterial growth. Rough finishes hide local defects such as stains, knots, ugly grains, fine cracks etc.
Smooth finishes are costly, require fine tools and superior techniques. Smooth finish often give poor bondage to preservatives and coatings but one requires much lesser quantity for coverage. Such finishes collect little dirt. Smooth finishes are not as susceptible to bacterial growth as the rough finishes. The timbers for smooth finish should have a fine grain pattern. Heart portions are much better for smooth finish then sap portions. Sap portions may however be finished fairly smooth, provided are immediately covered with moisture proof coating. Hardwoods usually provide smoother finish and of permanent type then soft woods. Woods with resinous or oily substances generally have smooth feel, however, if the substances are reactive or soluble in water or aliphatic solvents, may create problems during coating. Sisam and rosewood have oily or waxy face, which does not allow oil paints or varnish finish. Such woods need to be covered with very thin coatings based on solvent evaporation drying; like nitro cellulose lacquer. Timber surfaces are flame charred or singed to provide slightly darker to black tone to selected areas. The flame is either ‘cool’ capable of depositing carbon, or `hot’ to singe the surface.