Erosion of penetrating clear/stain finish on timber weatherboards

Cause

UV light deterioration

Repair

  • The first step is determining (if not known) the type of finish. Is it:

    • a film-forming stain or clear finish, where the coating sits on the surface, or

    • a penetrating stain, where the coating soaks into the timber surface?

  • film-forming coatings weather by peeling or flaking or becoming more opaque while penetrating stains weather by losing their intensity and general appearance.

  • where a building has eaves there will be a differential weathering and variation in colour and coating performance across the wall surface. Deterioration will be greater where the wall is not sheltered by eaves. This will result in: 

    • colour variations across the wall between sheltered and exposed areas

    • faster coating deterioration where the coating is exposed to sun (UV) and rain.

Film-forming clear finish or stain deterioration

  • film-forming clear or lightly pigmented coatings are vulnerable to ageing of the underlying timber and loss of coating adhesion under UV light from the sun
  • some timber producers do not recommend clear finishes for timber elements fully exposed to sun and weather
  • recoating of clear or pigmented film-forming stains is likely to be required every 6 months–2 years
  • scrape or sand off the existing coating to give an as-new timber surface. Recoating with the same type of product will restore appearance but frequent recoating will be required
  • major coating manufacturers have advice lines you can call for help and product recommendations for your specific circumstances.

Penetrating stain deterioration

  • wash down the surface with a low-pressure chemical wash to remove all dirt and any fungal growth. Do not waterblast as this will damage the timber. Once dry, apply a new coat of penetrating stain following the manufacturer’s instructions
  • recoating with a stain or finish with greater colour depth may produce less unevenness. Darker colours applied to timber bring other problems, however. They attract the heat and often lead to greater thermal expansion/contraction of timber during the day. This can shorten the lifespan of a coating.
  • penetrating stains will typically need to be reapplied every 2–4 years.

Cause

Moisture penetration and timber swelling

Repair

  • prepare the weatherboards as above, but ensure that the timber is allowed to dry completely before recoating.

Cause

Mould growth

Repair

  • prepare the weatherboards as above, but ensure that mould is treated and the timber is completely clean and dry before recoating.