Wood finishing

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 where typically it represents between 5 and 30 % of manufacturing costs.

Finishing is the final step of the manufacturing process that gives wood surfaces desirable characteristics, including enhanced appearance and increased resistance to moisture and other environmental agents. Finishing can also make wood easier to clean and influence other wood properties, for example tonal qualities of musical instruments and hardness of flooring. In addition, finishing provides a way of giving low-value woods the appearance of ones that are expensive and difficult to obtain.

Planning the finish (thinking ahead)

1. Design and end-use;
2. Wood characteristics - colour & texture;
3. Cost;
4. Mark-up, gluing, handling and storage;
5. Wood moisture content.

Basic wood finishing procedure

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. The resulting product is known as "fumed oak".

Types of finishes

There are three major types of finish:

• 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 may use solvents such as white spirits and naphtha as a base. Varnishes, linseed oil and tung oil are reactive finishes, meaning they change chemically when they cure, unlike evaporative finishes. This chemical change is typically a polymerisation, and the resultant material is less readily dissolved in solvents .

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

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 thinned with denatured alcohol as a finish and as a way to manipulate the wood's ability to absorb other finishes. The alcohol evaporates almost immediately to yield a finish that will attach to virtually any surface, even glass, and virtually any other finish can be used over it.

• 

  Linseed oil

• 

  Shellac

• 

  2:1 ratio of beeswax and carnauba wax

• 

  No finish

• 

  Molten bee wax

• 

  Shellac and linseed oil

• 

  Acrylic paint

• 

  Alkyd

• 

  Spar varnish

• 

  Tung oil

• 

  Tung oil and linseed oil

• 

  Acrylic varnish

	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	Difficult. 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
Pre-Cat lacquer	Transparent, all sheens from 5% to 90%	Good general protection against wet and dry heat.	Meets UK and European standards for "general use".	Uses toxic solvents. Spray booth is needed.	Spray application only.	Non-Reversible after 5 days	Excellent general furniture finish used extensively outside USA
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

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

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

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

• Wood stain
• Refinishing
• Distressing
• Danish oil

References

• 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

• Shellac Application
• Finishes on Antique Wood Furniture
• To Refinish or Not to Refinish (Antique Furniture)
• Finishing for First-Timers
• Oil Finishes
• Homeshop finishes that work