Maintenance & Care

Thermowood is low-maintenance by design - decades of rot resistance without any chemical treatment - but it is still a natural material. Here's how it weathers, and what you can do to influence how it looks over its service life.

The patina

Untreated Thermowood transitions from its original honey-brown color to a uniform silver-grey over roughly 18-36 months, depending on sun exposure. South- and west-facing walls grey faster. North-facing walls and soffits stay honey-brown longer, sometimes indefinitely in sheltered locations.

The patina is aesthetic only. Structural performance the 25-year rot resistance, the dimensional stability, the fungal resistance does not depend on coatings or color. A weathered silver Thermowood board is structurally identical to a fresh-milled one.

Many of our projects are specified explicitly to go silver. It's a beautiful, architectural color, and it has the advantage of being virtually maintenance-free.

Preserving the original color

If you want to hold the honey-brown original tone, a penetrating oil finish does the job. We recommend Cutek Extreme, which we stock and ship alongside material orders.

Application schedule

• First coat: within 30 days of installation, applied in dry weather
• South- and west-facing elevations: re-coat annually
• North- and east-facing elevations: re-coat every 2-3 years
• Soffits, sheltered elevations, and decks under cover: re-coat every 3-5 years

How to apply

Clean the surface first remove dust, debris, and any pollen or mildew. Apply oil with a lambswool applicator or soft brush, in the direction of the grain. Work small areas (10-20 sq ft at a time) and wipe excess with a lint-free cloth before it becomes tacky. Let cure 24-48 hours before any contact.

Most projects use approximately one gallon of oil per 250 sq ft of surface area for the first coat. Re-coats typically use half that.

Mixed finishes

Some architects specify a mixed approach oiled on the most prominent facades, untreated on back and side elevations. Over time the oiled areas retain the honey-brown tone while the untreated areas transition to silver. Done intentionally, this creates a textured visual effect that celebrates the wood's natural behavior.

Cleaning

Dirt, pollen, spider webs, and other surface contamination can be cleaned with warm water and a soft-bristle brush. Mild, non-ionic soap (like unscented dish soap) is fine for stubborn marks.

Never use a high-pressure washer. It lifts grain on any wood siding, including Thermowood. The result is a fuzzy, raised-grain surface that ages unevenly and holds more dirt.

For mildew spots in humid climates, a diluted oxalic acid solution (sold as "deck cleaner") is safe and effective. Rinse thoroughly and allow to dry before re-oiling if applicable.

Sauna-specific care

Aspen sauna benches and paneling should never be oiled or finished. The wood's low thermal conductivity the whole reason aspen is used in saunas depends on an open surface that can breathe.

Clean sauna aspen with water and a soft brush, after the room has cooled. Wipe benches dry after each use to prevent sweat absorption and staining. A light sanding every few years restores the surface if it becomes heavily discolored.

What to expect in the first year

Some homeowners are surprised by how quickly their Thermowood starts to grey in the first summer. This is normal and not a defect. If you've decided to let the wood weather naturally, expect:

• Weeks 1-8: original honey-brown tone, slight lightening on south elevations
• Months 3-6: visible greying on sun-exposed areas, honey retained in shade
• Months 12-4: uniform silver-grey on exposed areas, still partial tone in sheltered spots
• Year 3+: fully uniform silver patina across the facade

Warranty coverage

Thermosticks warrants against rot and decay for 25 years when installed per our published guidelines. Cosmetic weathering greying, minor checking, small surface cracks, and natural variation between boards is not a warranty condition. It's expected behavior of a real wood material and does not affect structural performance.

For warranty claims or questions about a specific condition on your installation, email sales@stickssupply.com with photos and we'll come back to you within five business days.