Finishing and maintaining thermally modified timber

Your three options

• Let it weather: no coating, and the timber will silver naturally. Zero maintenance, but expect some colour variation in the first year or two.
• Pigmented UV-protective oil or stain: keeps the brown tone longer and evens out colour. Reapply when the surface starts to look dry.
• Opaque coating: paint or solid stain for a coloured finish. Follow the coating manufacturer guidance for thermally modified timber.

Why it turns grey

Heat treatment darkens the wood all the way through, but it does not make it UV stable. Sunlight breaks down lignin at the surface, and rain washes the products away, leaving a grey layer. This is cosmetic and does not affect the stability or durability of the board beneath. How fast it happens depends on exposure: south and west elevations and unsheltered boards grey first, while areas under eaves can stay brown for much longer.

Choosing a product

• Clear oils: keep the natural look but give little UV protection, so the timber still greys beneath them. They suit interiors and sheltered areas.
• Pigmented oils and stains: the pigment is what blocks UV light. Darker, more heavily pigmented products hold colour longer than pale ones.
• Paint and solid stains: give full colour change and the best UV screening, but they form a surface layer that needs more careful preparation and eventual repainting.

Use a product made for exterior timber, and ask the maker whether it is suitable for thermally modified wood. Because the timber absorbs little, thin coats of a penetrating product work better than thick film coats.

Applying an oil or stain

1. Install the boards and ensure the surface is clean, dry and free of dust. Brush off any fine dust left from machining.
2. Test on an offcut or hidden area first, as thermally modified timber is less absorbent than untreated wood.
3. Apply thin coats with a brush or pad, following the grain, and wipe off any excess.
4. Coat all faces and end grain where practical, ideally before installation.
5. Allow to dry fully before exposing the surface to rain.

Decking and horizontal surfaces

Decking takes the heaviest wear, with direct sun, standing water and foot traffic. Expect it to need attention more often than vertical cladding. Make sure water can drain and air can circulate beneath, keep gaps between boards clear of debris, and coat cut ends. If you want to keep the brown colour on a deck, plan to refresh the finish regularly.

Inspecting and recoating

Check the surface once or twice a year. A coating is due when water no longer beads on the surface, or when the colour looks faded or patchy. Exposed elevations usually need attention before sheltered ones, so you can recoat in stages. Clean the boards, let them dry, and apply a fresh thin coat. Recoating before the finish has worn through is easier than starting again.

Restoring weathered boards

Greyed boards can be brought back. Clean them with soft water, a mild detergent and a brush, and allow them to dry fully. A light sand along the grain removes loose, weathered fibres and exposes fresher timber, although it will not restore the original deep colour all the way through. Then apply a pigmented finish. Sand only as much as needed, and wear a dust mask.

Cleaning

• Brush off dirt and leaves regularly, especially at the base of the cladding.
• Wash with soft water and a mild detergent and a soft brush.
• Avoid high-pressure washers, which can damage the surface fibres.
• Treat algae or green growth in damp, shaded areas with a suitable cleaner.

Common mistakes

• Using ordinary steel fixings, which can corrode and cause black staining.
• Skipping ventilation behind the cladding.
• Applying thick film-forming coatings that peel in sunlight.
• Leaving cut ends unprotected.
• Using exterior finishes inside a sauna. Inside, use only products the maker states are sauna-safe.

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Frequently asked questions