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. Americans also seem to be more casual in their approach to wood finishing, using products available from local hardware stores. In Britain supplies are more often ordered via the manufacturer representative and are for "Professional Use Only" without instructions appearing 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 |
Automated Wood Finish Application
Manufacturers who mass produce products implement automated flatline finish systems that run a on a conveyer 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
- DIYinfo.org's Wood Finishes Wiki - A practical and extensive guide to finishing wood around the house
- Choosing a Finish Article by Jeff Jewitt
- Finishes on Antique Wood Furniture - Many different finishes have been used to beautify and protect wood furniture through the centuries.
- Finishing for First-Timers Article by Bob Flexner.
- Painting Furniture Learn how you can antique paint furniture
- Oil Finishes Article by Bob Flexner on oil and oil-varnish blends
- To Refinish or Not to Refinish (Antique Furniture) - Collectors and experts have debated the question for many years.
- Homeshop finishes that work An article comparing and contrasting many popular finishes that can be applied by a DIY'er.
- To Refinish or not to Refinish? Hobbithouse Refinishing -- An essay providing guidance for the antique furniture owner with regard to refinishing, restoration and preservation.
- Complete Guide to Wood Finishing - You will learn how to get beautiful, professional results even if you have never tried your hand at wood finishing before.
- Advanced Equipment Systems - Examples of Automated Finishing Systems
How To Videos
Applying Waterbased Finishes YouTube Vid