Timber finishes - exterior

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Generally, the function of finishes on timber is two-fold, firstly to improve the durability of the building and secondly to add to the aesthetics. Unfinished, unprotected timber will inevitably weather as a result of gradual changes to its physico-chemical structure brought about by temperature and moisture content variations. The rate is generally slow, at about 6mm per century.

The weathering process leads to a slow breaking down and wearing away of surface fibres, change in colour and roughening of the surface. Under extreme conditions, timber may deform, check, split and pull away from fasteners. The extent of weathering will vary with timber species and ambient conditions.

Although unprotected timber has been used externally for centuries, the weathered effect is not always desirable. In most applications timber needs protection from the elements of water, cold, heat and ultra-violet light to promote a long service life. In addition to protection from the elements, finishes may provide a decorative effect.

The performance of exterior finishes depends on a wide range of factors that are discussed in detail in Design Guide 13, Finishing Timber Externally, available for download below. The local environment however, is a variable which needs special consideration and local experience can be of assistance. Advice should be sought from manufacturers or Timber Advisory Services.

The selection and correct application of the most appropriate finishing product is a most important component of the design and construction process.

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.

Factors Affecting Performance

The performance of external finishes is a significant factor affecting the service life of a building. However, fashion, new technologies, aesthetic improvements and changes in function often necessitate modification of the building, including its external envelope at more frequent intervals. In addition to these changes, the durability of an external finishing system on timber or timber products is influenced by a range of physical factors including those listed in the table below. Further information can also be found in the related document Design Guide 5, Timber Service Life Design Guide.

Substrates

Timber is essentially a variable substrate, even within a single species. The density, moisture content, absorbency, flexibility and the nature of its extractives may vary considerably. Some timbers are also more susceptible to dimensional change due to moisture or humidity variations. The finishing system must be sufficiently flexible to cater for this movement.

Table 1: Factors affecting performance of finishes

Factor

Affect on Performance

Substrate

Refer Table 2

Environmental Conditions

Dry temperate conditions favour very good performance compared with tropical, marine and severe industrial environments which provide relatively severe exposure conditions.

Pre-treatment

Use of preservative impregnation or water repellent preservative treatment before painting, together with putties and sealants where appropriate, will improve service life. 

Maintenance Schedule

Re-coating before major paint film breakdown occurs, results in improved durability at reduced cost.

Application Procedure

Adequate film build and attention to coating end-grain results in improved performance.

Building Design

 

Protection from elements using roof overhangs, verandahs, pergolas, etc., and avoiding water traps will promote improved performance. 

System choice

Correct choice of a system for each particular job and use of one manufacturer's products in each system will contribute to the long term performance of the finish. 

Uniformly fine textured timbers have better paint holding characteristics than coarse textured species. The latter, however, respond better to penetrating stain applications than conventional paint systems.

The manner in which timber is cut from a log affects the orientation of the annual rings in the piece and this, in turn, affects its paint holding properties.

If a board is quarter-sawn (edge-grained) as opposed to back-sawn (flat grained), its dimensional stability is better under variable moisture conditions and it is less likely to result in surface checking to the detriment of the paint system.

It should be noted however, that most parcels of timber are supplied as a mix of back-sawn and quarter-sawn pieces. Finishing systems are developed to cater for this variation. Discolouration or blistering of the finish may be caused by water-soluble extractives of the heartwood such as gum or resin. This effect is accentuated by heat and is more prevalent on the side of a structure receiving maximum sunlight. Kiln drying of the timber will usually "set" these extractives, removing or minimising the problem.

Some timbers have a higher percentage of aromatic oils, which contribute to their characteristic pleasant odours but may present problems with paint discolouration and inhibit drying if left unprepared before painting. Cleansing with a solvent may be necessary.

Other timber characteristics that affect paint performance are knots, bark, gum pockets and insect damage. Knots will in most cases absorb more of the finish than side-grain and have a greater potential to check and exude resin or gum. Where plywood or glued laminated timber is used, an exterior timber grade must be specified together with a conventional finishing system for timber.

Exterior grade hardboard has a smooth uniform surface finish and is usually supplied pre-primed, ready for finishing.

Table 2: substrate Affect on performance

Substrate

Variable

 

Affect on Performance

 

Species

Differs between and within species.

Density

The heavier, denser timbers are less prone to moisture uptake than lighter species

Dressed timber

Better performance with conventional paint systems. 

Sawn timber

Better performance with solid and natural stains.

Timber edges

Sharp edges create stress in paint films, causing failure. Arrised or rounded edges permit film movement with minimal stress. 

Back-sawn timber versus Quarter-sawn timber

Quarter-sawn softwood boards generally have better dimensional stability than back-sawn, and therefore better paint holding properties. For hardwood the difference is minimal.

Unseasoned (green) timber

 

Greater likelihood of blistering and peeling occurring particularly with solvent borne coatings. Stains are more effective. Seasoned timber Best condition for painting

Weathered versus unweathered timber

 

Surface deterioration of timber fibres due to weather can result in poor paint adhesion unless corrective sanding is carried out. Stains may perform better than conventional finishes if sanding is omitted.

Extractives

May cause topcoat discolouration or blistering unless surface extractives are removed with a solvent wash prior to priming. 

Heartwood versus Sapwood

Heartwood has a higher staining potential than sapwood but is ideally suited to painting with exterior timber stains. 

Earlywood versus Latewood

 

Cracking on latewood is more likely to occur as the solvent borne paint embrittles on extended exposure.

Smooth texture versus coarse texture

 

Smooth texture surfaces have better paint holding potential than coarse texture ones. Use of stains on the latter gives good performance. 

Gum pockets

Can lead to resin exudation andstaining unless pre-treated and sealed.

Aromatic oils

Can lead to drying retardation and staining if surface oils are not removed. 

Knots

Possible premature cracking, staining and resin exudation can occur unless treated with knotting varnish or manufacturers recommend treatment.

Bark

Can lead to premature failure of all film-forming finishes if not removed. 

Insect Damage

Premature failure may occur if not treated or filled before finishing.

Moisture

Content

 

Normal range 10-15% - higher levels detrimental, particularly for solvent borne finishes where moisture vapour permeability is low. 

Design and Construction

The two major factors which affect the performance of the finish, and subsequently that of the timber, are moisture and sunlight. Any design features which minimise possible moisture ingress to the substrate or exposure to high angle incident sunlight will be beneficial.

In Australia (Southern Hemisphere) the north facing exposure is most severely affected by sunlight, with the severity of exposure changing with the latitude of the site itself, varying from 15 degrees near Cape York to about 45 degrees near Tasmania. Guidance on the design of structures to provide shade protection is given in NSB 120, Shadow Angles.

Some design steps which can be taken to assist in excluding moisture and protecting from U.V. are:

  • The use of wide eave overhangs, verandas, pergolasetc.
  • The use of vertical rather than horizontal siding.
  • The use profiles with rounded arrises.
  • The fitting of drip caps over doors and windows.
  • Avoidance or minimisation of joins in horizontal sidings likelyto allow moisture ingress.
  • Provision of adequate ventilation of roof space and subfloorspace
  • The use of recommended nail size and pattern forvarious types of timber cladding
  •  Selective use of boards that show defects, such as knots. When they are used, use them for the least exposed areas.

For information regarding the specific finish types download Design Guide 13, Finishing Timber Externally available below.

Finish Types

There is a wide range of finish types with selection being governed by the degree of protection required and the appearance sought. The table below summarises the various finishes. Advice is also available from manufacturers and reference can be made to the standard AS/NZS 2311 - Guide to the Painting of Buildings.

Table 3: Exterior finishes and applications

Paint Type

Comments

 

Conventional opaque systems -water and solvent borne

 

Best suited for use on dressed and seasoned timber. Wide colour ranges withuse of tinting systems, good durability.

Natural timber finishes - semitransparent and opaque

Best suited for use on sawn or textured durable timbers. More frequent butmuch simpler maintenance.

Water Repellent Preservatives

Temporary pre-treatment to prevent soiling and deterioration during construction but have limited life if not overcoated.

Wood Primers

Brush application is recommended. Solvent borne - better penetration, but slower drying. Water borne - better long term flexibility, quicker overcoating and easier clean up. 

Undercoats

Good opacity and bridging properties. Increase system film build and durability. Solvent borne - best over chalky surfaces. Water borne - faster drying, better colour qualities and easier clean up. 

Finishing Coats - Gloss

Solvent borne have sharper gloss, good flow, opacity and durability. Water borne have slightly lower initial gloss, better gloss retention, poorer flow but better film flexibility after aging. Water borne system applied to primed timber should prove superior durability to solvent borne.

Finishing Coats - Semi-Gloss

Comments as for gloss but gloss level lowered. Slight reduction in durability but minimise appearance of surface imperfections.

Finishing Coats - Flat and Low Sheen

Solvent borne - rarely used for exterior. Water borne - good durability and colour.

 

Natural Finishes - Opaque or Solid Stains

 

Generally solvent borne. Enhance both timber grain and texture and help maintain timber colour. Care required in application to avoid lap joint colour difference. 

Natural Finishes - Semi-Transparent Stains

Obscures grain but enhances texture. Have better durability than semitransparent stains. Water borne "acrylic" solid stains generally give superior performance to solvent borne finishes particularly over knotty timbers. 

Natural Finishes - Clear

Enhances timber appearance but requires a commitment to more regularmaintenance with frequent inspections. Solvent borne clears have improved with use of U.V. absorbers specific to resin type used. Water borne clears, or slightly tinted versions, have durability somewhat equivalent to opaque systems. Water borne clears are more flexible and thermoplastic.

Film Forming versus Penetrating Finish

All finishes provide a coating on the surface and to some extent fill voids in the microscopic surface structure. Film forming and penetrating finishes vary in thickness and in the finished shape of the surface.

Film forming finishes appear as a distinct layer and usually display a plane surface. Penetrating finishes on the other hand follow the contours of the timber surface, providing thicker deposits in the troughs and thinner coatings over the peaks.

Due to the greater coating thickness, film forming finishes are more resistant to wear, but penetrating finishes have the ability to accommodate differential movement of timber due to moisture variations. The advantages of penetrating finishes include:

  • Natural appearance
  • No peeling or blistering
  • Suited to sawn textured surfaces
  • No trapping of moisture in timber
  • Easily applied and renewed

 

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