Wood finishing

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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.[1][2]
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[3] and influence other wood
properties, for example tonal qualities of musical instruments and hardness of flooring. [4][5] In addition,
finishing provides a way of giving low-value woods the appearance of ones that are expensive and
difficult to obtain.

Contents
[hide]

 1Planning the finish (thinking ahead)[6]

 2Sanding
o 2.1Removing Larger Defects
 3Bleaching and Removal of Stains
 4Colouring Wood
 5Basic wood finishing procedure
 6Types of finishes
 7Comparison of different clear finishes
 8Automated wood finishing methods
o 8.1Hangline approach
o 8.2Towline approach
 9See also
 10References
 11External links

Planning the finish (thinking ahead)[6][edit]

Finishing of wood requires careful planning to ensure that the finished piece looks attractive,
performs well in service and meets safety and environmental requirements.[6]Planning for finishing
begins with the design of furniture.[6] Care should be taken to ensure that edges of furniture are
rounded so they can be adequately coated and are able to resist wear and cracking. Careful
attention should also be given to the design and strength of wooden joints to ensure they do not
open-up in service and crack the overlying finish.[7] Care should also be taken to eliminate recesses
in furniture, which are difficult to finish with some systems, especially UV-cured finishes. [8]
Planning for wood finishing also involves thinking about the properties of the wood that you are
going to finish, as these can greatly affect the appearance and performance of finishes, and also the
type of finishing system that will give the wood the characteristics you are seeking. [6] For example,
woods that show great variation in colour between sapwood and heartwood or within heartwood may
require a preliminary staining step to reduce colour variation.[9] Alternatively, the wood can be
bleached to remove the natural colour of the wood and then stained to the desired colour. [10][11] Wood’s
that are coarse textured such oaks and other ring-porous hardwoods may need to be filled before
they are finished to ensure the coating can bridge the pores and resist cracking. The pores in ring
porous woods preferentially absorb pigmented stain and advantage can be taken of this to highlight
the wood’s grain.[7] Some tropical woods such as rosewood (Dalbergia nigra), cocobolo (Dalbergia
retusa) and African padauk (Pterocarpus soyauxii) contain extractives such as quinones or stilbenes
retard the curing of unsaturated polyester and UV-cured acrylate coatings and other finishing
systems should be used with these species.[12][13][14]

Planning for wood finishing also involves thinking the production process influences finishing.
Careful handling of the wood is needed to avoid dents, scratches and soiling with dirt. [6] Wood should
be marked for cutting using pencil rather than ink, however, avoid hard or soft pencil; HB is
recommend for face work; 2H for joint work.[6] Care should be taken to avoid squeeze-out of glue
from joints because the glue will reduce absorption of stain and finish. Any excess glue should be
carefully removed to avoid further damage to the wood. Wood’s moisture content affects staining of
wood.[15] Changes in wood moisture content can result in swelling and shrinkage of wood which can
stress and crack coatings. Both problems can be avoided by stored wood indoors in an environment
where it can equilibrate to a recommended moisture content (6 to 8%) that is similar to that of the
intended end use of the furniture.[7]
Finally, consideration needs to be given to whether the finished wood will come into content with
food, in which case a food safe finish should be used,[16] local environmental regulations governing
the use of finishes,[17] and recycling of finished wood at the end of its life.[18]

Sanding[edit]
Sanding is carried out before finishing to remove defects from the wood surface that will affect the
appearance and performance of finishes that are subsequently applied to the wood. [7] These defects
include cutter marks and burns, scratches and indentations, small glue spots and raised grain.
[7]
Sanding should not be used to eliminate larger defects such as gouges, and various forms of
discolouration.[7] Other techniques are used to remove these defects (see below).
Large open ‘pores’ in Eucalyptus sieberi

Pore distribution in southern beech

The key to preparing a defect free surface is to develop a sanding schedule that will quickly
eliminate defects and leave the surface smooth enough so that tiny scratches produced by sanding
cannot be seen when the wood is finished.[19] A sanding schedule usually begins with sandpaper that
is coarse enough to remove larger defects (typically 80 or 100 grit, but sometimes higher if the
surface is already quite smooth), and progresses through a series of sandpaper grades that
gradually remove the sanding scratches created by the previous sanding steps.[19] A typical sanding
schedule prior to wood finishing might involve sanding wood along the grain with the following
grades of sandpaper, 80, 100, 120, 150 and finishing with 180 and sometimes 220 grit. [7] The precise
sanding schedule is a matter of trial and error because the appearance of a sanded surface depends
on the wood you are sanding and the finish that will subsequently be applied to the wood.
[19]
According to Nagyszalanczy,[19] coarse grained woods with large pores such as oak hide sanding
scratches better than fine grained wood and hence with such species it may be possible to use 180
or even 150 grit sandpaper as the final step in the sanding schedule.[19] Conversely, sanding
scratches are more easily seen in finer grained, harder woods and also end-grain, and hence, they
require finer sandpaper (220 grit) during the final sanding stage.[19] The sandpaper selected for the
final sanding stage affects the colour of stained wood, and therefore when staining is part of finishing
avoid sanding the wood to a very smooth finish.[7] On the other hand, according to Nagyszalanczy if
you are using an oil-based finish, it is desirable to sand the wood using higher grit sandpaper (400
grit) because oil tends to highlight sanding scratches.[19]
Scanning Electron Microscopy of 120 grit aluminum oxide paper

Scanning Electron Microscopy of 180 grit aluminum oxide paper

Profilometry image of Yellow birch after sanding

Sanding is very good at removing defects at wood surfaces, but it creates a surface that contains
minute scratches in the form of microscopic valleys and ridges, and also slivers of wood cell wall
material that are attached to the underlying wood.[20][21][22] These sanding ridges and slivers of wood
swell and spring-up, respectively, when sanded wood is finished with water-based finishes, creating
a rough fuzzy surface. This defect is known as grain raising. It can be eliminated by wetting the
surface with water, leaving the wood to dry and then lightly sanding the wood to remove the ‘raised
grain’.[19]
Profilometry image of Yellow birch after sanding and wetting

Removing Larger Defects[edit]

Larger defects that interfere with wood finishing include dent, gouges, splits and glue spots and
smears.[7] These defects should also be removed before finishing, otherwise they will affect the
quality of the finished furniture or object. However, it is difficult to completely eliminate large defects
from wood surfaces.
Removing dents from wood surfaces is quite straightforward as pointed out by. [7] Add a few droplets
of demineralized water to the dent and let it soak in. Then put a clean cloth over the dent and place
the tip of a hot iron on the cloth that lies immediately above the dent, taking great care not to burn
the wood. The transfer of heat from the iron to the wood will cause compressed fibres in the dent to
recover their original dimensions. As a result the dent will diminish in size or even disappear
completely, although removal of large dents may require a number of wetting and heating cycles.
The wood in the recovered dent should then be dried and sanded smooth to match the surrounding
wood.
Gouges and holes in wood are more difficult to repair than dents because wood fibres have been
cut, torn and removed from the wood. Larger gouges and splits are best repaired by patching the
void with a piece of wood that matches the colour and grain orientation of the wood under repair.
[7]
Patching wood requires skill, but when done properly it is possible to create a repair that is very
difficult to see. An alternative to patching is filling (sometimes known as stopping). [7][23] Numerous
coloured fillers (putties and waxes) are produced commercially and are coloured to match different
wood species. Successful filling of voids in wood requires the filler to precisely match the colour and
grain pattern of the wood around the void, which is difficult to achieve in practice. Furthermore, filled
voids do not behave like wood during subsequent finishing steps, and they age differently to wood.
Hence, repairs to wood using fillers may noticeable.[7] Therefore filling is best used with opaque
finishes rather than semitransparent finishes, which allow the grain of the wood to be seen.

Macro photo of 120 grit sandpaper

Macro photo of 220 grit sandpaper

Macro photo of 320 grit sandpaper

Glue smears and droplets are sometimes present around the joints of furniture. They can be
removed using a combination of scraping, scrubbing and sanding.[7] These approaches remove
surface glue, but not the glue beneath the wood surface. Sub-surface glue will reduce the absorption
of stain by wood, and may alter the scratch pattern created by sanding. Both these affects will
influence the way in which the wood colors when stains are used to finish the wood. To overcome
this problem it may be necessary to locally stain and touch-up areas previously covered by glue to
ensure that the finish on such areas matches that of the surrounding wood. [7]

Bleaching and Removal of Stains[edit]

Wood surfaces are occasionally affected by various organic and inorganic stains.[24] Sometimes such
stains enhance the colour and appearance of wood.[24] For example, oak wood affected by the beef-
steak fungus has a deep rich, attractive, brown colour[25] and there is no reason to remove the stain
from the wood prior to finishing. The same applies to spalted wood whose attractive appearance is
again caused by fungi. On the other hand some fungal stains and those caused by the reaction of
iron with wood can disfigure wood.[24] These stains can be removed from wood using bleach.[26]
[27]
Bleaches are also occasionally used to reduce the difference in colour between lighter sapwood
and heartwood and also colour variation within heartwood.[7] Such bleaching make it easier to
achieve a uniformly coloured wood when the wood is subsequently coloured with pigmented stains
and dyes[7] (see below). Furthermore, the natural colours of wood fade when wood is exposed to
sunlight, and more permanent colours can be created by bleaching wood to remove its natural
colour and then re-colouring the wood using artificial, light-fast, stains. [7]
The bleaches used to remove unwanted fungal stains from wood include two-part peroxide bleach
and solutions of sodium hypochlorite.[27] The former is particularly effective at removing the natural
colour of wood before it is recoloured with pigmented stains or dyes.[27] Oxalic acid is particularly
effective at removing iron stains from wood.[27]

Colouring Wood[edit]
Wood can be stained to change its colour or left unstained before application of lacquer, or other
types of top-coats. Staining should enhance the appearance of wood by reducing colour variation
between and within sapwood and heartwood. It also provides a way of giving bland looking woods
such as poplar, the appearance of prized furniture woods such as ebony, mahogany or walnut.
Wood can be stained using dyes or pigmented finishes. These finishes are available in a wide
variety of colours, many of which are not part of the natural colour palette of wood, for example,
blues and greens. Pigmented stains tend to highlight the grain (and also sanding scratches),
whereas dyes do not have this effect and are more transparent. Wood can also be coloured by
exposing it to chemicals that react with the wood to form coloured compounds. Chemical staining of
wood is rarely carried out because it is easier to colour wood using dye or pigmented stain,
however, ammonia fuming is a chemical staining method that is still occasionally used to darken
woods such as oak that contain a lot of tannins. Staining of wood is difficult to control because some
parts of the wood absorb more stain than others, which leads to problems such as blotchiness and
streaking. For this reason, as pointed out by Flexner, many people prefer to omit the staining step
when finishing wood.

Basic wood finishing procedure[edit]

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.[28] The
resulting product is known as "fumed oak".

Types of finishes[edit]
There are three major types of finish:[29]

 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[edit]

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.