Wood finishing - AbsoluteAstronomy.com

Wood finishing refers to the process of embellishing and/or protecting the surface of a wooden material. The process starts with surface preparation, either by sanding

Sandpaper

Sandpaper, also known as glasspaper, is a heavy paper with abrasive material attached to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother , to remove a layer of material...

by hand (typically using a sanding block

Sanding block

A sanding block is a block used to hold sandpaper. In its simplest form, it is a block of wood or cork with one smooth flat side. The user wraps the sandpaper around the block, and holds it in place...

or power sander

Sander

A sander is a power tool used to smooth wood and automotive or wood finishes by abrasion with sandpaper. Sanders have a means to attach the sandpaper and a mechanism to move it rapidly contained within a housing with means to hand-hold it or fix it to a workbench. Woodworking sanders are usually...

), scraping

Card scraper

A card scraper is a woodworking shaping and finishing tool. It is used to manually remove small amounts of material and excels in tricky grain areas where hand planes would cause tear out. Card scrapers are most suitable for working with hardwoods, and can be used instead of sandpaper...

, or planing

Plane (tool)

A hand plane is a tool for shaping wood. When powered by electricity, the tool may be called a planer. Planes are used to flatten, reduce the thickness of, and impart a smooth surface to a rough piece of lumber or timber. Planing is used to produce horizontal, vertical, or inclined flat surfaces on...

. Imperfections or nail holes on the surface may be filled using wood putty or pores may be filled using wood filler

Wood filler

Wood putty, also called plastic wood, is a substance used to fill imperfections, such as nail holes, in wood prior to finishing. It is often composed of wood dust combined with a binder that dries and a diluent , and, sometimes, pigment. Pore fillers used for large flat surfaces such as floors or...

. Often, the wood's colour is changed by staining

Wood stain

A wood stain consists of a colorant suspended or dissolved in a 'vehicle' or solvent. The suspension agent can be water, alcohol, petroleum distillate, or the actual finishing agent...

, bleach

Bleach

Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...

ing, ammonia

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

fuming and a number of other techniques. Some woods such as pine

Pine

Pines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...

or cherry do not take stain evenly, resulting in "blotching". To avoid blotching, a barrier coat such as shellac

Shellac

Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes , which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish...

or "wood conditioner" is applied before the stain. Gel

Gel

A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...

stains are also used to avoid blotching.

Once the wood surface is prepared and stained, a number of coats of finish may be applied, often sanding between coats. Commonly used wood finishes include wax

Wax

thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

, shellac

Shellac

, drying oil

Drying oil

A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air. The oil hardens through a chemical reaction in which the components crosslink by the action of oxygen . Drying oils are a key component of oil paint and some varnishes...

s (such as linseed oil

Linseed oil

Linseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...

or tung oil

Tung oil

Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree . As a drying oil, tung oil hardens upon exposure to air. The resulting coating is transparent plastic-like and is exploited in most of its applications, which include wood finishing and the...

), lacquer

Lacquer

In a general sense, lacquer is a somewhat imprecise term for a clear or coloured varnish that dries by solvent evaporation and often a curing process as well that produces a hard, durable finish, in any sheen level from ultra matte to high gloss and that can be further polished as required...

, varnish

Varnish

Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss...

, or paint

Paint

Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...

. Other finishes called "oil finish" or "Danish oil

Danish Oil

Danish oil is a wood finishing oil, made of either Tung oil or Polymerized Linseed oil.Danish oil is a hard drying oil which provides a tough, water-resistant finish. It can be used as a finish, or as a primer or sealer on bare wood before applying paint or varnish.When used, Danish oil provides...

" are actually thin varnishes with a relatively large amount of oil and solvent. Water-based finishes can cause what is called "raising the grain" where surface fuzz emerges and requires sanding down.

Finally the surface may be polished

Polishing

Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...

or buffed using steel wool

Steel wool

Steel wool, also known as wire wool, is a bundle of strands of very fine soft steel filaments, used in finishing and repairing work to polish wood or metal objects, and for cleaning household cookware....

, pumice

Pumice

Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...

, rotten stone

Rotten stone

Rotten stone, sometimes spelled as rottenstone, also known as tripoli, is fine powdered rock used as a polishing abrasive in woodworking. It is usually weathered limestone mixed with diatomaceous, amorphous, or crystalline silica...

and other polishing or rubbing compounds depending on the shine desired. Often, a final coat of wax can be applied over the finish to add a slight amount of protection.

French polish

French polish

French polishing is a wood finishing technique that results in a very high gloss surface, with a deep colour and chatoyancy. French polishing consists of applying many thin coats of shellac dissolved in alcohol using a rubbing pad lubricated with oil...

ing is not polishing as such, but a method of applying many thin coats of shellac using a rubbing pad, yielding a very fine glossy finish.

Special tools used to apply wood finishes include rags, rubbing pads, brush

Brush

A brush is a tool with bristles, wire or other filaments, used for cleaning, grooming hair, make up, painting, surface finishing and for many other purposes. It is one of the most basic and versatile tools known to mankind, and the average household may contain several dozen varieties...

es, and spray guns

Spray painting

Spray painting is a painting technique where a device sprays a coating through the air onto a surface. The most common types employ compressed gas—usually air—to atomize and direct the paint particles. Spray guns evolved from airbrushes, and the two are usually distinguished by their size and the...

. The processes involved and the terminology for the materials used are quite different in Britain than the processes and terms used in the USA. For instance, the process of replicating the look and feel of traditional French polished wood is more commonly done in the UK by "pulling over" precatalysed lacquer, within 24 hours of spraying, whereas in the U.S. a "rubbed" finish is more common.

Types of finishes

There are basically three types of finish :

Evaporative
Reactive
Coalescing

Wax is an evaporative finish because it is dissolved in turpentine or petroleum distillates to make the familiar soft paste. After these distillates evaporate all that's left is the wax.

Reactive finishes use solvents such as white spirits and naphtha. Oil varnishes and linseed oil are reactive finishes which change chemically when they cure, unlike evaporative finishes. At cure, the solvent/thinner evaporates and the resins cluster tighter together, and then a chemical reaction occurs causing the resins to cross link in a different chemical format—like loose scaffolding that suddenly bolt together. Scuff sanding is necessary between layers of cured finish so that subsequent applied layers have something to grip on to effectively. The solvent won't re-dissolve the cured film, eg, white spirits does not soften cured oil based varnish.

Note that pre-catalysed and post catalysed "lacquers" (aka acid catalysed lacquers) are reactive finishes. The term lacquer is, in this sense, used inconsistently from product to product.

The oil based varnishes dry from the top down by reacting with oxygen. The catalysed lacquers dry from the bottom up (which is like the evaporative finishes) and the solvents migrate upwards to the film surface and then out leaving behind molecules that then crosslink.

Tung oil and linseed oil are reactive finishes that cure by reacting with oxygen, but do not really form film finishes when cured.

Water based finishes generally fall into the coalescing category.

Comparison of different clear finishes

Clear finishes are intended to make wood look good and meet the demands to be placed on the finish. Choosing a clear finish for wood involves trade-offs between appearance, protection, durability, safety, requirements for cleaning, and ease of application. The following table compares the characteristics of different clear finishes. 'Rubbing qualities' indicates the ease with which a finish can be manipulated to deliver the finish desired.
Shellac should be considered in two different ways. It is used as a finish and as a way to manipulate the wood's ability to absorb other finishes by thinning it with denatured alcohol. The alcohol evaporates almost immediately to yield a finish that is completely safe but shellac will attach itself to virtually any surface, even glass, and virtually any other finish can be used over it.

	Appearance	Protection	Durability	Safety	Ease of Application	Reversibility	Rubbing Qualities
Wax Wax thumb\|right\|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...	Creates shine	Short Term	Needs frequent reapplication	Safe when solvents in paste wax evaporate	easy, needs sanding	Can easily be removed with solvents	Needs to be buffed
Shellac Shellac Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes , which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish...	Some yellow or orange tint, depending on grade used	Fair against water, good on solvents except alcohol	Durable	Safe when solvent evaporates, used as food and pill coating	French polishing difficult technique to master.	Completely reversible using alcohol	Excellent
Nitrocellulose lacquer	Transparent, good gloss	Decent protection	Soft and somewhat durable	Used toxic solvents Good protection is needed, especially if painted	Requires nice equipment. Kick-on products also available	Completely irreversible	Excellent soft finish
Conversion varnish	Transparent, good gloss	Excellent protection against many substances	Hard and durable	Uses toxic solvents, including toluene. Breathing protection is needed	Requires spray equipment. Used in professional shops only	Difficult to reverse	Excellent hard finish
Linseed oil Linseed oil Linseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...	Yellow warm glow, pops grain¹, darkens with age	Very little	Fairly durable, depending on number of coats	Relatively safe, metallic driers are poisonous	Easy, apply with rags and wipe off. Takes relatively long time to dry	Needs sanding out as oil is absorbed	None
Tung oil Tung oil Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree . As a drying oil, tung oil hardens upon exposure to air. The resulting coating is transparent plastic-like and is exploited in most of its applications, which include wood finishing and the...	Warm glow, pops grain¹, lighter than linseed	Water resistant	Fairly durable, depending on number of coats	Relatively safe, metallic driers are poisonous	Easy, apply with rags and wipe off. Faster to dry than linseed oil	Needs sanding out as oil is absorbed	None
Alkyd varnish Alkyd An alkyd is a polyester modified by the addition of fatty acids and other components. They are derived from polyols and a dicarboxylic acid or carboxylic acid anhydride. The term alkyd is a modification of the original name "alcid", reflecting the fact that they are derived from alcohol and...	Not as transparent as lacquer, yellowish/orange tint	Good protection	Durable	Relatively safe, uses petroleum based solvents	Brush or spray. Brushing needs good technique to avoid bubbles & streaks	Can be stripped using paint removers	Fair
Polyurethane varnish	Transparent, many coats can look like plastic	Excellent protection against many substances, tough finish	Durable after approx. 30 day curing period	Relatively safe, uses petroleum based solvents	Application requires some level of skill	Can be stripped using paint removers	Bad, coats do not meld leading to white rings if rubbing out cuts through coat
Water-based polyurethane	Transparent	Good protection. Newer products (2009) also UV stable	Durable after approx. 10 day curing period	Safer than oil-based, fewer volatile organic compounds (VOCs)	Brush or spray. Fast drying demands care in application techniques	Can be stripped using paint removers	Bad, coats do not meld leading to white rings if rubbing out cuts through coat
Oil-varnish mixes	Similar to oils unless many coats applied, then takes on characteristics of varnishes	Low, but more than pure oil finishes	Fairly durable, depending on number of coats (archaic product, little used with the availability of modern finishes)	Relatively safe, uses petroleum based solvents	Easy, apply with rags and wipe off. Faster to dry than linseed oil	Needs sanding out as oil is absorbed	None unless many coats applied

¹ accentuates visual properties due to differences in wood grain.

Automated Wood Finishing Methods

Manufacturers who mass produce

Mass production

Mass production is the production of large amounts of standardized products, including and especially on assembly lines...

products implement automated flatline finish systems. A flatline finishing system is a long series of processing stations that may include sanding, dust removal, staining, sealer and topcoat applications. As the name suggests, the primary part shapes are flat. Liquid wood finishes are applied via automated spray guns in an enclosed environment or spray cabin. The material then can enter an oven or be sanded again depending on the manufacturer’s setup. The material can also be re-entered into the assembly line to apply another coat of finish or continue in a system that adds successive coats depending on the layout of the production line.

Two very common methods of automating the wood finishing process are the hangline approach and the towline approach.

In the hangline approach, wood items being finished are hung by carriers or hangers that are attached to a conveyor system

Conveyor system

A conveyor system is a common piece of mechanical handling equipment that moves materials from one location to another. Conveyors are especially useful in applications involving the transportation of heavy or bulky materials...

that moves the items overhead or above the floor space. The conveyor itself can be ceiling mounted, wall mounted or supported by floor mounts. A simple overhead conveyor system can be designed to move wood products through several wood finishing processes in a continuous loop. The hangline approach to automated wood finishing also allows the option of moving items up to warmer air at the ceiling level to speed up drying process.

The towline approach to automating wood finishing uses mobile carts that are propelled by conveyors mounted in or on the floor. This approach is useful for moving large, awkward shaped wood products that are difficult or impossible to lift or hang overhead, such as four-legged wood furniture. The mobile carts used in the towline approach can be designed with top platens that rotate either manually or automatically. The rotating top platens allow the operator to have easy access to all sides of the wood item throughout the various wood finishing processes such as sanding, painting and sealing.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.