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 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 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 technique difficult to master. 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. 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 Yellow warm glow, pops grain1, darkens with age Very little Fairly durable, depending on number of coats Relatively safe, metallic driers are poisonous, rags may spontaneously combust Easy, apply with rags and wipe off. Takes relatively long time to dry Needs sanding out as oil is absorbed None
Tung oil 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 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 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
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 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, rarely used due to 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
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.

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.

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]