Exterior Timber Finishes

Bare Timber vs Finished Timber

Some designers prefer timber to weather naturally, but this choice comes with performance trade-offs.

Decay Conditions

• For timber to decay due to fungal attack, four conditions must be satisfied:
• Moisture content
  • 0-20% - Attack will not occur
  • 20-60% - Attack may occur
  • 60%+ - Attack unlikely due to insufficient oxygen
• Oxygen must be present
  • Completely submerged or saturated timber is rarely attacked
  • Timber 600mm or more below the ground is rarely attacked
• Temperature
  • <5ºC, attack is slowed
  • 5-40ºC, fungus grows. 25-40ºC fungus thrives
  • 40ºC+, fungus cannot survive
• Food
  • From the timber itself, particularly sapwood, which is higher in sugars and carbohydrates

When is treatment required?

The chart below provides a good first approximation for a preservative treatment pathway:

Figure 1: Preservative treatment pathway chart

Notes:

• This chart should only be used as a first approximation
• Only sapwood, in both hardwoods and softwoods, can consistently be effectively preservative treated, unless the timber is incised. Incising the timber means that the surface of the timber (usually heartwood, which is naturally more resistant to penetration) has been mechanically pierced with many small cuts or slits using specialised machinery before preservative treatment, to allow transmission of preservative treatment.
• The sapwood of some species, i.e. cypress and Douglas fir cannot be effectively penetrated.
• Plantation softwoods and some hardwood species have wide sapwood bands and are ideal for preservative treatment, particularly in round form.

Types of Exterior Timber Finishes

Factors Affecting Finish Performance

Timber Species

• Open-grain species (e.g. Silvertop Ash) may require multiple coats or priming
• Dense hardwoods (e.g. Spotted Gum) can repel coatings or delay drying
• Treated timbers (e.g. LOSP) may inhibit adhesion of some finishes

Surface Preparation

• Timber should be clean, dry, and sanded before coating
• Remove mill glaze on dressed hardwoods (by light sanding or mild etching)
• Moisture content must be below 15% to ensure proper film formation

Orientation and Exposure

• North- and west-facing walls experience harsher UV and drying cycles
• Exposed horizontal members (e.g. handrails) will require more frequent recoating

Factory Finishing vs Site Application

Factory-applied finishes offer better consistency and quality control, but require careful handling and protection during transport and install.

No coating lasts forever. All finishes will eventually degrade under UV and moisture cycling, particularly on exposed horizontal or north-facing surfaces.

Figure 2: Typical maintenance schedule for various treatment types

🛈 Always clean surfaces prior to recoating. For film-forming systems, light sanding may be required to avoid peeling.

Film-forming finishes

Film-forming finishes, such as paints, clear surface coatings and heavy- bodied stains appear as a layer on the surface of the timber, visually creating a smooth surface and do not penetrate significantly into the surface of the timber. Experience has shown that these finishes have, at times, been associated with premature decay, Success will require very regular maintenance to ensure there is no break in the film, however slight, and also the removal of the coating between applications.

Decay under a film finish:

Figure 3: Decay under a film-forming treatment. Stubbersfield Newsletter September 2025

Good detailing and timber fibre selection:

Figure 4: Good detailing and timbre fibre selection. Stubbersfield Newsletter September 2025

Detailing for Coating Longevity

Coatings will fail faster if timber is poorly detailed. Best practices include:

• Round or arris all sharp edges to reduce stress concentrations
• Use drip edges on horizontal members
• Avoid unventilated cavities - trapped vapour causes blistering
• Seal all end grain prior to installation

Figure 5: Light and dark coatings in a durability study

Light coloured finishes are preferred to dark colours as they are less prone to checking and distortion due to intense cycles of heating and cooling.

For decks requiring slip-resistance: 

For new applications a Slip- resistant surface can be achieved by installing a board with a rough sawn, lightly sanded/brushed face. For existing applications, the face can be roughened with a specialised brushing process. Alternatively, add a slip-resistant additive to the deck finishing product. Some coating manufactures have products with anti-slip particles already included, while others have particles that can be added to standard coating products. Refer to the coating product manufacturer for more information. However, experience has shown that this will require a very high level of maintenance as noted above under film forming finishes. For an in depth discussion refer to Guide #48 - Slip Resistance and Wood Pedestrian Surfaces.

Preservative-treated timber may affect the performance of certain finishes:

Always check manufacturer datasheets for:

• Dry time over treated timber
• Adhesion over specific preservatives
• Breathability in multi-layered systems

Application Best Practice

• Ensure timber surfaces are dry and clean before applying any finish.
• Prime and coat all surfaces prior to installation, including end-grain and cut edges.
• Re-seal cut ends on site immediately after cutting.
• Apply finishes in line with manufacturer instructions, including curing times and minimum coat numbers.

Maintenance Cycles

Maintenance requirements vary by finish type, exposure, and orientation:

• North-facing, unshaded elements may require recoating every 1-2 years.
• Protected elements (eaves, verandahs) may last 3-4 times longer before recoating.
• Film-forming finishes require more regular inspection; once compromised, moisture can become trapped and accelerate decay.

Regular inspection and cleaning prolong the service life of finishes. Remove dirt, debris, and fungal growth to maintain surface protection.

Risks of Poor Coatings

• Low-quality or poorly maintained acrylic paints on low-durability timber have been shown to accelerate decay, sometimes by a factor of two.
• Oils or water repellents alone may require very frequent reapplication in exposed conditions.

Summary

• Exterior timber finishes are essential for moisture resistance, UV protection, and appearance
• Choose finish type based on exposure, aesthetic, species, and maintenance expectations
• Penetrating systems are breathable and easy to maintain, but require more frequent recoating
• Film-forming systems last longer but must be applied over stable, detailed surfaces
• Factory finishes improve performance but need careful site handling
• All finishes must be reapplied over time - schedule recoats to prevent surface degradation
• Proper detailing (rounding edges, sealing joints) extends finish life