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A wood stain consists of a colorant suspended or dissolved in an agent or solvent. The suspension agent can be water, alcohol, petroleum distillate, or the actual finishing agent (shellac, lacquer, varnish, polyurethane, etc.). Colored or 'stained' finishes, like polyurethane, do not penetrate the pores of the wood to any significant degree and will disappear when the finish itself deteriorates or is removed intentionally.
Pigments and dyes are largely used as colorants. The difference between the two is in the size of the particles. Dyes are microscopic crystals that dissolve in the vehicle and pigments are suspended in the vehicle and are much larger. Dyes will color very fine grained wood, like cherry or maple, which pigments will not. Those fine-grained woods have pores too small for pigments to attach themselves to. Pigments contain a binder to help attach themselves to the wood.
The type of stain will either accentuate or obscure the wood grain. Most commercial stains contain both dye and pigment and the degree to which they stain the appropriate wood is mostly dependent on the length of time they are left on the wood. Pigments, regardless of the suspension agent, will not give much color to very dense woods but will deeply color woods with large pores (e.g. pine). Dyes are translucent and pigments are opaque.
Gel stains are more akin to paint and have little penetrating ability.
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 dyeing 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.
Comparison with paint and varnish
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 the top or at the surface. Stains that employ metallic pigments such as iron oxides usually are more opaque; first because metallic pigments 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 colour or pigment and is designed to form a surface film. Some products are marketed as a combination of stain and varnish.
Siding stain is one variety of wood stain with very high viscosity (others can be quite thin). Effectively, siding stains are paints that do not cover as well and do not form a hard film. They are designed to penetrate better and contain binders that are softer and more flexible, allowing them to last longer than harder, more brittle paints. Siding stain protects against solar radiation especially UV radiation, water, fungus including mildew, and insects. Different siding stains are distinguished by the appearance they impart to wood. Certain solvent-based or oil-based siding stains contain small amounts of paraffin wax, which cannot be painted over, although re-staining is still possible.
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 absorbent, thus will accept 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. Different wood species stain differently—the overall color and shade is a result of a combination of the stain and properties of the wood. For example, although medium-to-dark stains tend to look blotchy on maple, they get deeper and more glowing on cherry, with a more consistent coloration. 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 composed of metal oxides, namely titanium dioxide and zinc oxide do not penetrate well and remain on the surface. In such cases, wear easily reveals unstained wood. 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.
In certain cases you need to clean the wood or remove existing stains prior to staining the wood with a commercial stain in order to avoid damaging the wood. This can be the case for both unfinished and finished wood.
Cleaning stained wood
There are special consideration when cleaning stained wood. One of the most common stains is water stains on stained wood. Techniques to remove the water stains have been documented that use a hot iron to remove the water stain.
Special caution should be used when trying to remove stains from a stained wood to avoid damaging the original stain. This is especially important with antiques.
- Michael Dresdner, The Woodfinishing Book