Wood finishing

Wood finishing

Wood finishing refers to the process of embellishing and/or protecting the surface. The process starts with surface preparation, either by sanding 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 colour is changed by staining, bleaching, ammonia fuming and a number of other techniques. Some woods such as pine or cherry do not take stain evenly, resulting in "blotching". To avoid blotching, a barrier coat such as shellac or "wood conditioner" is applied before the stain. Gel 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, shellac, drying oils (such as linseed oil or tung oil), lacquer, varnish, or paint. Other finishes called "oil finish" or "Danish Oil" 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 or buffed using steel wool, pumice, rottenstone 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 polishing 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.

Different tools used to apply wood finishes include rags, rubbing pads, brushes, and spray guns. 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 US a "rubbed" finish is more common.

'''==Comparison of different clear finishes as used in America== Wood varnish is a great way to make your work look nice. Choosing a clear finish for wood involves trade-offs between appearance, protection and durability, safety, ease of application, reversibility, and rubbing qualities. The following table compares the characteristics of different clear finishes. "Rubbing qualities", a term of art, indicates the ease with which the finish can be sanded between coats. It does not indicate a method of application such as brush or rag.

Appearance Protection Durability Safety Ease of Application Reversibility Rubbing Qualities
Wax Creates shine Great Lasts forever Safe when solvents in paste wax evaporate Applied with steel wool, 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 difficult technique to master. Completely reversible using alcohol Excellent
Nitrocellulose lacquer Transparent, good gloss Good protection Hard and durable Uses toxic solvents, including toluene. Breathing protection is needed, especially if sprayed Requires spray equipment. Brush-on products also available Completely reversible using lacquer thinner Excellent hard finish
Conversion lacquer Transparent, good gloss Excellent protection against many substances Hard and durable Uses toxic solvents, including toluene. Breathing protection is needed, especially if sprayed Requires spray equipment. Used in professional shops 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 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 Very little 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 Relatively safe, uses petroleum based solvents Brushing needs good technique to avoid bubbles & streaks Can be stripped with difficulty using paint removers Bad, coats do not meld leading to white rings if rubbing out cuts through coat
Water-based polyurethane Transparent, may give cold bluish tinge to wood Good protection Durable Safer than oil-based, fewer volatile organic compounds (VOCs) Brush or spray. Brushing needs good technique to avoid bubbles & streaks Can be stripped with difficulty 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 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 Finish Application

Manufacturers who mass produce products implement automated flatline finish systems that run a on a conveyor belt that first begin by being sanded, then dust is removed, and the wood finish is applied via spray gun. 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 depending on the manufacture.

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

See also

Wood Finishing Articles & Essays

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