How to Finish Thermo-Aspen Sauna Benches: Wax, Oil, and Care

What Thermory recommends for its boards

Thermory’s June 2026 catalog recommends treating all wooden surfaces before use with a breathable wood protector. The stated purpose is to guard against moisture and dirt and make cleaning easier. Walls and floor grids get sauna wax or another suitable coating, and the product must suit sauna conditions Thermory catalog, p. 39.

For all wooden surfaces, Thermory USA’s April 2026 installation guide names Supi Saunavaha, “or similar wood coating specifically made for sauna use,” and says all finishing must follow the finish maker’s instructions Thermory USA guide, p. 4. Thermory USA’s sauna brochure describes Supi Saunavaha as a natural wood wax that penetrates the wood to protect against dirt and water Thermory USA brochure, p. 16.

For benches, the catalog lists pre-waxed SHP bench boards in aspen, in black, white, and translucent finishes, and says Thermory highly recommends waxing sauna materials for longevity. Its SmartS series PLATE bench board, made in thermo-aspen or alder, comes unfinished, and Thermory recommends oiling it to add protection and extend its service life Thermory catalog, pp. 12, 20. Bench board profiles are on the profiles page.

What the ThermoWood® Handbook recommends for benches

Mineral oil. The English edition puts it in one line: “Mineral oil is recommended for interior walls, ceilings and benches in saunas” ITWA handbook (English), p. 33. More broadly, the handbook says ThermoWood® takes the same kinds of finish as standard timber, including paint, lacquer, oil, and wax, in water-based or solvent-based form ITWA handbook, p. 33.

Paraffin oil counts as a mineral oil: the USDA Wood Handbook describes “mineral (or paraffin) oil” as a nondrying oil from petroleum USDA Wood Handbook, p. 16-31.

Read the handbook line as a recommendation, not a rule. It does not say an unfinished sauna bench is wrong, and it gives no application rate or schedule. It advises asking finish manufacturers about the right treatment and how to maintain it. It also explains why a finish sits well on this wood: ThermoWood® is resin-free and swells and shrinks only slightly with moisture, so coatings applied to it do not crack ITWA handbook, p. 33.

Why linseed oil and kitchen oils are the wrong fit

The handbook’s reason for ruling out linseed oil is short: it enables fungal growth on ThermoWood® ITWA handbook, p. 33. It names only linseed oil, so read the warning narrowly rather than stretching it to every natural oil.

The USDA Wood Handbook explains how the oil types differ, in its section on finishes for wood that touches food. Drying oils such as tung, linseed, and walnut cure into a barrier to liquid water and may need several weeks to cure. Nondrying oils, vegetable and mineral, soak into wood without curing and still improve water resistance. On a sauna bench, the useful difference is what microbes can feed on. Vegetable oils such as olive, corn, peanut, and safflower are food for mildew and bacteria and can turn rancid, while paraffin oil and wax do not turn rancid and do not support microbes USDA Wood Handbook, p. 16-31.

For how moisture and mold interact in a hot room, see can mold grow in a sauna.

Getting a finish to hold on thermally modified wood

Acidity and low water absorbance can keep some finishes from adhering, according to the handbook. Thermal modification lowers the wood’s pH: about 4 for ThermoWood® products, against 4.5 to 5 for a corresponding standard product ITWA handbook, pp. 12, 33. The process page covers the kiln process behind the change.

The handbook singles out some water-based products, and it says “some,” not all ITWA handbook, p. 33. Use the product your board’s maker names, and ask the finish maker whether it suits thermally modified wood.

Surface texture matters too. A surface sanded with P100 paper may hold a finish better than a planed one, and brushed surfaces have worked well. A sawn surface can be too rough because of splinters ITWA handbook, p. 33.

The same acidity is why the handbook calls for acid-resistant or stainless steel fasteners in thermally modified timber ITWA handbook, p. 12. See stainless fasteners for sauna paneling for the grades.

Keep the finish breathable, and seal the cut ends

Treat the faces with a breathable finish, and seal only the cut ends. Thermory’s word for its recommended protector is “breathable” Thermory catalog, p. 39. The USDA Wood Handbook classes mineral oil as a nondrying oil that soaks into wood without curing, apart from the sealers and drying oils that cure into a barrier to liquid water USDA Wood Handbook, p. 16-31.

The room air is the reason. Like standard timber, ThermoWood® is hygroscopic and evens out swings in indoor humidity, though less so because of its lower equilibrium moisture content, the moisture level wood settles at in a given climate. A treatment that seals the wood too effectively stops moisture moving between the room air and the wood ITWA handbook, p. 30.

Cut ends are the exception. The handbook says the cut ends of any timber, ThermoWood® included, should be sealed with an end-grain sealer made for the purpose. That considerably reduces water uptake through the ends and reduces end cracking as the wood dries ITWA handbook, p. 33.

When you trim bench boards to length, seal the fresh ends. Thermory asks that any finish suit sauna conditions and be applied to its maker’s instructions, so hold an end-grain product to the same test Thermory catalog, p. 39. For trimming and storage, see cutting, sanding, and storing thermo-aspen.

A bench care routine, step by step

This routine follows Thermory’s care instructions for its sauna wood. Where a finish label gives different directions for that product, follow the label.

Before the first session

Thermory’s catalog sets these steps Thermory catalog, p. 39:

• Clean the wood of dust with a sauna detergent or another cleaner suitable for wood, and let it dry thoroughly.
• Apply a breathable wood protector, sauna wax, or another coating suitable for sauna conditions, and follow its maker’s instructions.
• If you oil an unfinished bench board, the oil’s label sets how long to let it stand before you wipe off the excess. The directions for Harvia’s paraffin oil for saunas are one example.

During and after every session

Thermory USA’s guide lists these habits Thermory USA guide, p. 4:

• Throw water only on the heater, not on the walls or benches.
• If the bench is too hot, sit on a seat cover or a dry towel, and avoid sitting in wet swimwear.
• Wipe the bench with a clean damp cloth. Skip running water, and do not let water pool on the wood.
• Take seat covers and wet towels off the bench boards.
• Dry the room. Thermory recommends leaving the heater on for at least half an hour with the air vent open and the door and windows closed.

At least once a year

Thermory’s guide and catalog give the yearly steps Thermory USA guide, p. 4 Thermory catalog, p. 39:

• Re-treat the wooden surfaces with a product meant for the purpose, and more often if the sauna is used often.
• Clean seats, walls, and ceilings with a soft brush or cloth and a cleaner made for the job, following its package directions.
• For stains or stubborn dirt, sand the entire length of the bench with fine-grained sandpaper, then treat the sanded areas again.
• Check the bench fastenings and supports, and tighten screws that heat and humidity cycles have loosened.

For the handbook’s emissions and indoor-air findings, see what the VOC tests found. More care answers are on the questions page.