UV / sun bleaching

All clear-finished timber floors change colour under UV. Most Australian eucalypts darken (red-browns deepen, ambers turn amber-brown). A few species bleach toward grey. Understanding the direction and rate lets you plan furniture layouts, window treatments and recoat cycles.

Mechanism

Ultraviolet light breaks down lignin at the wood surface and photo-oxidises both the timber's natural extractives and the coating resin. WoodSolutions TDG-13 notes 'UV cannot reach the surface of the timber and break it down' under pigmented paints, but clear film and oil finishes allow UV through; film-forming clear coats can themselves become brittle through prolonged UV exposure and break down. Water-based acrylic systems are typically more flexible than solvent-based systems, and light colours are generally more durable than darker colours because they do not absorb as much heat from sunlight.

Risk profile by species

SpeciesRiskNote & source
Jarrah moderate-darkening Jarrah is a classic red Australian hardwood; Bona White is explicitly 'not recommended for use on darker or red timbers'. High-pigment species with care required on coverage to avoid colour variation.Source: species_primer_matrix.json (Bona Intense TDS + Bona Primers DL Brochure)
Blackbutt moderate-darkening High-tannin hardwood; tannin bleed is possible under UV exposure with white primers. Bona Intense is specifically recommended to minimise tannin bleed.Source: species_primer_matrix.json (Bona Primers DL Brochure)
Spotted Gum low-medium-colour-shift WoodSolutions datasheet: 'compared to other Australian hardwoods, spotted gum is a minimal staining timber as it is less prone to bleed-through of tannins than other species'.Source: spotted-gum-woodsolutions.html
Brushbox moderate-darkening Bona Intense TDS names Brushbox alongside Jarrah as requiring 'particular care... to observe the coverage rate of 8 m2/L' because colour variation is possible under coating.Source: Bona_Intense_Primer_TDS
Ironbark high-darkening FPC WA species information PDF notes Ironbark bark has tannin content of 40% or more; sap and heartwood pigments produce strong tannin bleed risk under waterborne coatings.Source: fpc-species-information-western-australia.pdf

Mitigations

  • traffic-hd
    Specify a waterborne polyurethane topcoat rather than a solvent-based one. WS TDG-13 notes water-based acrylic systems are typically more flexible than solvent-based systems and that light colours are generally more durable than darker colours because they do not absorb as much heat from sunlight.
    Source: ws-tdg-13-finishing-timber-externally.pdf p.43
  • prime-intense
    Use Bona Intense primer on high-tannin and dark-red species so the resulting colour is deeper and more uniform; observe the 8 m2/L coverage rate on Jarrah and Brushbox to avoid colour variation.
    Source: Bona Intense Primer TDS (Feb 2022) + Bona Primers DL Brochure Ed4 09/24
  • For pigmented appearances, film-forming paints conceal texture and prevent UV reaching the timber; but they become brittle through prolonged UV exposure and can break down over time.
    Source: ws-tdg-13-finishing-timber-externally.pdf p.43
  • Plan window treatments (drapes, blinds, external awnings) and rotate furniture / rugs quarterly during the first 2 years while fresh timber is most photo-reactive. Timber Queensland TDS-29 warns that local effects (locality or building design) can significantly affect in-service exposure.
    Source: tq-tds-29-moisture-effects-on-in-service-performance.pdf p.2

Bona product guidance

  • traffic-hd
    'Clear, non-yellowing formula which enhances the natural beauty of a timber floor'. 2K waterborne polyurethane with advanced polymer technology.
    Source: Bona_TrafficHD_TDS_AU.pdf (2025-03-25) p.1
  • prime-intense
    Recommended on Blackbutt and Tallowwood to minimise tannin bleed; suitable on Jarrah / Brushbox with 8 m2/L coverage control.
    Source: Bona Intense Primer TDS p.1 + Bona Primers DL Brochure Ed4
  • prime-white
    Not recommended for use on darker or red timbers, and not recommended for high-tannin timbers (Blackbutt tannin bleed possible under Improved Formula).
    Source: Bona_White_Improved_Formula_TDS_AU.pdf + Bona_Primers_DL_Broch.pdf

Gaps in the corpus (no claim made)

  • Quantitative UV darkening rates per species (e.g., ΔE per 1000 hours QUV) are not in the corpus — only qualitative behaviour.
  • No explicit UV-resistance rating (e.g., QUV-B panel hours) published in Bona Traffic HD TDS that we could cite.
Sources