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Timber finishes - interior

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Timber 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, scraping, or planing. Imperfections or nail holes on the surface may be filled using wood putty or pores may be filled using wood filler, the wood's colour is changed by a variety of techniques that typically involve staining, bleaching or ammonia fuming. Note that some woods such as pine or cherry do not absorb stain evenly, which results in "blotching". To avoid blotching, a barrier coat such as shellac or a conditioner should be applied before the stain. Gel stains can also be 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 a linseed oil or tung oil) lacquer 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 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.

Timber finishes are constantly being improved and new options developed. Check with your  reseller or manufacturer for up-to-date details. If you have a product or information that you think should be included on this page, please contact us.. 

Types of Finishes

Evaporative

Evaporative finishes use alcohol, acetone and nitro-cellulose lacquer thinners as solvents and thinners. Nitro-cellulose lacquers and shellac fall into this group. The solids are soft and string-like in solution but as the solvents evaporate they lock together in a solid mass like dried spaghetti. Successive layers burn in to one another and form a contiguous whole. The solvent will re-soften the film, e.g. lacquer thinners will soften cured lacquer, and alcohol softens cured shellac.

Cellulose polishes and thinners and lacquer and lacquer thinners are in the same family of finishes. Lacquer thinners, also known as cellulose thinners do come in different flavours, e.g., 'hot' or 'fast' thinners and 'cool' or 'slow' thinners depending on how the formulation is meant to perform.

Wax is an evaporative finish because it is dissolved in turpentine or petroleum distillates to make the familiar soft paste. Once these distillates evaporate all that's remaining is the wax.

Reactive

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, a chemical reaction then 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, i.e., white spirits do not soften cured oil based varnish.

Note that pre-catalysed and post catalysed "lacquers" (also known as 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.

Coalescing
Water based finishes generally fall into the coalescing category.

Comparison of different clear finishes
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

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 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

Timber stains

A timber stain consists of a colorant suspended or dissolved in a 'vehicle' or solvent. The suspension agent can be water, alcohol, petroleum distillate, or the actual finishing agent (shellac, lacquer, varnish, polyurethane, etc.). Colored or 'stained' finishes, like polyurethane, do not penetrate the pores of the wood to any significant degree and will disappear when the finish itself deteriorates or is removed intentionally.

Two types of colorants are used, pigments and dyes. The difference is in the size of the particles. Dyes are microscopic crystals that dissolve in the vehicle and pigments are suspended in the vehicle and are much larger. Dyes will color very fine grained wood, like cherry or maple, which pigments will not. Those fine-grained woods have pores too small for pigments to attach themselves to. Pigments contain a binder to help attach themselves to the wood.

The type of stain will either accentuate or obscure the wood grain and neither is superior to the other. Most commercial stains contain both dye and pigment and the degree to which they stain the appropriate wood is mostly dependent on the length of time they are left on the wood. Pigments, regardless of the suspension agent, will not give much color to very dense woods but will deeply color woods with large pores (eg. oak). Dyes are translucent and pigments are opaque.

Gel stains are more akin to paint and have little penetrating ability.

For specific information on colour ranges and product types please consult your local manufacturer or timber advisory service.

The "Timber Service Life Design Guide" is available for download below. Chapter 6 provides a detailed discussion of timber finishes.

Related Links:

Timber Finishes - Exterior

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