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Wood stain, a sub-category of paint, consists of a pigment suspended in a "vehicle" of solvent and binding agent (alkyd, linseed oil, acrylic, polyurethane, lacquer, or resin). It is formulated to impart or transport the pigment into the pores of the surface rather than creating a film on top of the surface. Stain is composed of the same three primary ingredients as paint (pigment, solvent, and binder) but is predominantly pigment (or dye) and solvent with little binder. Much like the dying or staining of fabric, wood stain is designed to add color to the substrate of wood and other materials while leaving the substrate mostly visible. Transparent varnishes or surface films are applied afterwards. In principle, stains do not provide a surface coating or film. However, because the binders are from the same class of film-forming binders that are used in paints and varnishes, some build-up of film occurs. The initial application of any paint or varnish is similarly absorbed into the substrate, but because stains contain lower amounts of binder, the binder from a stain resides mainly below the surface while the pigment remains near or at the surface. Stains that employ metallic pigments such as iron oxides usually are more opaque; first because metallic pigment are opaque by nature, but also because the particles of which they consist are much larger than organic pigments and therefore do not penetrate as well. Most wood stains for interior uses (e.g. floors and furniture) require further application of varnish or finish for protection and gloss. Stains are differentiated from varnishes in that the latter has no added color or pigment and is designed to form a surface film. Some products are marketed as a combined stain and varnish.
Stains range from very "thin," that is, low in viscosity, to siding stains that are extremely thick. Effectively, siding stains are paints that do not "cover" as well and do not form a hard film. Siding stains are designed to penetrate better than paint, provide protection against sunlight or UV damage, prevent water from penetrating, prevent mildew or fungal decay, and slow down insect attacks. The binders in siding stains are designed to be softer and more flexible and thus last longer than harder, more brittle paints. Finally, the differences between the multitude of siding stains, and brands thereof, which one encounters while trying to decide between them are governed by the ultimate appearance which they give the wood. Be advised, however, that certain solvent-based or oil-based siding stains contain small amounts of paraffin wax and cannot be painted over in the future, though they can be re-stained.
Applying stains can be very easy or very difficult depending on the type of substrate, type of stain, and the ambient conditions. Fresh, "green" lumber accepts stain poorly, while aged wood absorbs stains relatively well. Porosity of wood can vary greatly, even within the same piece of wood. End grain and bias-cut grain are far more absorptive, thus will absorb more pigment and will darken considerably in those areas. The hard ring may absorb differently from the soft ring. The characteristic medullary rays in oak will absorb much less and remain mostly blonde. Woods that have been heavily subjected to paint strippers or washed down with detergents or solvents will have an increased open grain and accept substantially more stain than normal. Woods from different species of trees can have huge variations in how well they take stain. Stains that are fast drying will be difficult to apply in hot weather or in direct sunlight. Stains that are slow-drying will be difficult to work with in damp and cold conditions due to a greatly lengthened evaporation and curing period. New lumber, such as pine, can have waxlike sealants put on at the mill that will prevent proper staining; stripping or sanding the surface may be required. White stains are invariably metal oxides, namely titanium dioxide and zinc oxide, and inherently do not penetrate the wood well. They remain on the surface and if wear occurs the unstained wood is easily revealed. They are also fairly opaque. Typically, a thorough final sanding of the wood with one suitable grit will "even out" the absorption of the stain. White stain on a bare softwood or oak floor might require a final 'prep' sanding by hand with an orbital/vibrating sander with 80 or 100 grit, whereas certain hardwoods might be orbitally or hand sanded with 220 grit and higher for a darkish organic stain on furniture. Though it is not as durable, "garnet" is by far the preferred sandpaper for hand-sanding bare wood; first because it is a sharper and faster cutting grit, and second because it does not impart color from the grit, as aluminum oxide or silicon carbide can do. Other methods include "rubbing" with rottenstone etc. See Wood finishing.
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