Wood Finishing

Editor: wilconix

Introduction

Wood finishing refers to the process of embellishing and/or protecting the surface of wooden objects.

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 so-called "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 even the terminology for the materials used are quite different in Britain from those 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, whereas in the US a "rubbed" finish is more common. Also, Americans seem to be more casual in their approach to wood finishing, using products available from local hardware stores whereas in Britain supplies are more often ordered via the manufacturer's representative and are for "Professional Use Only" with no instructions on the tin.

Comparison of different clear finishes as used in America

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 Very little Does not last and needs to be reapplied frequently Safe when solvents in paste wax evaporate Applied with rag, needs buffing 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 grain, 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 grain, 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

References:

http://en.wikipedia.org/wiki/Wood_finishing

• 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

Categories: Woodworking