Wood finishing refers to the process of embellishing and/or protecting the surface of a wooden material. 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, rotten stone 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.
Special 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 U.S. a "rubbed" finish is more common.
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There are basically three types of finish [1]:
Wax is an evaporative finish because it is dissolved in turpentine or petroleum distillates to make the familiar soft paste. After these distillates evaporate all that's left is the wax.
Reactive finishes use solvents such as white spirits and naphtha. Oil varnishes and linseed oil are reactive finishes which change chemically when they cure, unlike evaporative finishes. At cure, the solvent/thinner evaporates and the resins cluster tighter together, and then a chemical reaction occurs causing the resins to cross link in a different chemical format—like loose scaffolding that suddenly bolt together. Scuff sanding is necessary between layers of cured finish so that subsequent applied layers have something to grip on to effectively. The solvent won't re-dissolve the cured film, eg, white spirits does not soften cured oil based varnish.
Note that pre-catalysed and post catalysed "lacquers" (aka acid catalysed lacquers) are reactive finishes. The term lacquer is, in this sense, used inconsistently from product to product.
The oil based varnishes dry from the top down by reacting with oxygen. The catalysed lacquers dry from the bottom up (which is like the evaporative finishes) and the solvents migrate upwards to the film surface and then out leaving behind molecules that then crosslink.
Tung oil and linseed oil are reactive finishes that cure by reacting with oxygen, but do not really form film finishes when cured.
Water based finishes generally fall into the coalescing category.
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 difficult technique to master. | Completely reversible using alcohol | Excellent |
| Nitrocellulose lacquer | Transparent, good gloss | Decent protection | Soft and somewhat durable | Used toxic solvents Good protection is needed, especially if painted | Requires nice equipment. Kick-on products also available | 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 | 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 (VOCs) | 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 |
| 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, little used with the 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 |
1 accentuates visual properties due to differences in wood grain.
Manufacturers who mass produce products implement automated flatline finish systems. A flatline finishing system is a long 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 re-entered into the assembly line to apply another coat of finish or continue in a system that adds successive coats depending on the layout of the production line.
Two very common methods of automating the wood finishing process are the hangline approach and the towline 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.[2]
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.[3]
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