Wood finishing

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A worker sprays a urethane finish onto a timber

Wood finishing refers to the process of refining or protecting a wooden surface, especially in the production of furniture.

Basic wood finishing procedure[edit]

Wood finishing starts with sanding either by hand, typically using a sanding block or power sander, scraping, or planing. Imperfections or nail holes on the surface may be filled using wood putty or pores may be filled using wood filler. Often, the wood's color is changed by staining, bleaching, or any of a number of other techniques.

Once the wood surface is prepared and stained, the finish is applied. It usually consists of several coats of wax, shellac, drying oil, lacquer, varnish, or paint, and each coat is typically followed by sanding.

Finally, the surface may be polished or buffed using steel wool, pumice, rotten stone or other materials, depending on the shine desired. Often, a final coat of wax is applied over the finish to add a degree of protection.

French polishing is a finishing method of applying many thin coats of shellac using a rubbing pad, yielding a very fine glossy finish.

Ammonia fuming is a traditional process for darkening and enriching the color of white oak. Ammonia fumes react with the natural tannins in the wood and cause it to change colours.[1] The resulting product is known as "fumed oak".

Types of finishes[edit]

There are three major types of finish:[2]

  • Evaporative
  • Reactive
  • Coalescing

Wax is an evaporative finish because it is dissolved in turpentine or petroleum distillates to form a soft paste. After these distillates evaporate, a wax residue is left over.

Reactive finishes use solvents such as white spirits and naphtha. Oil varnishes and linseed oil are reactive finishes, meaning they change chemically when they cure, unlike evaporative finishes. The solvent evaporates and a chemical reaction occurs causing the resins to undergo a change. This change prevents solvents from dissolving reactive finishes.

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

Water based finishes generally fall into the coalescing category.

Comparison of different clear finishes[edit]

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 Dull, even sheen unless spit polished Short Term Needs frequent reapplication Safe when solvents in paste wax evaporate Difficult on bare wood Diffucult. Solvents thin wax causing it to penetrate deeper. Sanding creates heat. Scraping recommended Needs to be buffed for low sheen, buffed with damp pad for high gloss
Shellac From virtually clear (super blond)to a rich orange (garnet) Fair against water, poor solvent protection Durable Safe when solvent evaporates, used as food and pill coating Clogs spray equipment. Quick solvent flash time makes brushing difficult. Ox or badger/skunk hair brush recommended. Easy to pad, however French Polish is difficult Completely reversible using alcohol Excellent
Nitrocellulose lacquer Transparent, good gloss Decent protection Soft and somewhat durable Uses toxic solvents. Good protection is needed, especially if painted Requires equipment. Reversible with proper solvents 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
Boiled Linseed oil Yellow warm glow, pops grain1, darkens with age Very little Little Relatively safe, metallic driers are poisonous, rags may spontaneously combust Easy, but cure time can be upward of 30 days Difficult. All saturated wood needs to be removed (planing/sanding/scraping) Dries hard, can be buffed to high gloss
Tung oil Warm glow, pops grain1, lighter than linseed Water resistant Poor Relatively safe when fully cured. Pure tung oil contains no metallic dryers. Many products labeled tung oil are oil/varnish blends Difficult. Sanding is required between coats and a partial cure is necessary. Very long finishing schedule for sufficient amount of coats Difficult. All saturated wood needs to be removed (planing/sanding/scraping) Dries hard, can be buffed to high gloss
Alkyd varnish 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 Slight ambering, comes in a variety of sheen Excellent protection against many substances, tough finish Durable after approx. 7 day curing period Relatively safe, uses petroleum based solvents Very easy when thinned and wiped on. Also brushes and sprays well Can be stripped using paint removers Easy to rub out with steel wool or synthetic pads to reduce sheen. Because it is soft, it cannot be buffed to a high gloss
Water-based polyurethane Transparent Good protection. Newer products (2009) also UV stable when noted Durable after approx. 10 day curing period Safer than oil-based, fewer volatile organic compounds Brush or spray. Fast drying demands care in application techniques Can be stripped using paint removers Excellent. Acrylic finishes are very hard and can be buffed to an extreme gloss. Use a release agent
2-Part polyurethane Transparent Stronger protection than regular polyurethane varnish Durable once cured, generally less than an hour low or free of VOCs, nonreactive when cured generally sprayed, equipment must be cleaned of any mixed product immediately Irreversible Sands easily. Sanding not needed between coats
Oil-varnish blends (i.e. Danish Oil, Teak oil, "tung oil finish") Enhances natural figure like a drying oil, but easier to apply and more protective. Low, but more than pure oil finishes Fairly durable but not recommended for heavy use items such as table tops Relatively safe, uses petroleum based solvents Easy, apply with rags and wipe off. Too many applications can result in sticky build up Difficult. All saturated wood needs to be removed (planing/sanding/scraping) Dries hard. can be buffed to a matte finish or to a gloss. Often top coated with paste wax for extra protection
Epoxy resin Thick, high-gloss, and transparent. Some formulations can cloud or yellow with UV exposure High level of protection Flexible and durable Safe when cured Easy pour-on application for flat surfaces, difficult to apply evenly on more complicated shapes Cleanable with acetone when liquid. Irreversible once cured flexibility makes sanding difficult but possible

1 accentuates visual properties due to differences in wood grain.

Automated wood finishing methods[edit]

Manufacturers who mass-produce products implement automated flatline finish systems. These systems consist of a 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 recycled through the line to apply another coat of finish or continue in a system that adds successive coats depending on the layout of the production line. The systems typically used one of two approaches to production.

In this hangline approach, wood items being finished are moved through various finishing stages on a conveyor system

Hangline approach[edit]

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

In this Towline method, mobile carts move large furniture through various finishing stages on a conveyor system.

Towline approach[edit]

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.

See also[edit]

References[edit]

  1. ^ "Fuming white oak". 
  2. ^ http://www.ukworkshop.co.uk/forums/difference-between-lacquer-and-varnish-t39510.html
  • Michael Dresdner (1992). The Woodfinishing Book. Taunton Press. ISBN 1-56158-037-6
  • Bob Flexner (1994). Understanding Wood Finishing: How to Select and Apply the Right Finish. Rodale Press ISBN 0-87596-566-0

External links[edit]