Why Treat Thermally Modified Cladding with Osmo 420?

Both Thermowood Pine and Thermo Ayous are thermally modified, which means they don’t need a decorative coating to make them suitable for exterior use. If they’re left untreated, however, exposure to sunlight and the weather will gradually cause both timbers to weather to a silver-grey colour. If you want to retain more of the timber’s original warm colour, Osmo 420 Clear UV-Protection Oil can be used on exterior timber to help protect it against UV exposure. And because Thermowood Pine and Thermo Ayous have quite different natural appearances, applying exactly the same oil produces two noticeably different results.

Osmo 420 on Thermowood Pine

Thermowood Pine is a thermally modified Nordic softwood with visible knots, grain and natural variation. Before treatment, it has a warm, creamy to mid-brown colour. Once we applied Osmo 420, the colour became noticeably richer and warmer. The oil also brought out more of the natural grain and knots in the timber.

Rather than hiding the character of Thermowood Pine, the treatment emphasises it. So if you like timber where you can still see the knots, grain and natural variation, treating Thermowood Pine with Osmo 420 gives it a warmer, richer finish while retaining that character.

Osmo 420 on Thermo Ayous

Thermo Ayous starts with a very different appearance. It’s a lightweight thermally modified hardwood with considerably fewer knots and a much cleaner, more uniform face. When Osmo 420 is applied, the colour deepens to a warmer brown, but the timber retains that smooth and consistent appearance. The result is therefore quite different from the Thermowood Pine. Where the oil highlights the knots and grain in the Pine, on the Thermo Ayous it creates a richer, more even finish across the board. 

Thermowood Pine vs Thermo Ayous with Osmo 420

Put the two treated boards side by side and the difference is easy to see.

Thermowood Pine + Osmo 420:

Warmer and richer, with the grain and knots becoming more prominent. It’s the more characterful of the two finishes.

Thermo Ayous + Osmo 420:

A warmer, deeper brown with a cleaner and more uniform appearance. The timber’s contemporary look remains even after treatment.

Neither finish is necessarily better than the other, they’re simply different. If you prefer natural variation and visible timber character, Thermowood Pine may give you the look you’re after. If you prefer a smoother, more consistent finish with very few knots, Thermo Ayous may be more suitable.

What Is the Difference Between Thermowood Pine and Thermo Ayous?

Although this test is about how the two timbers look after being treated with Osmo 420, there are some important differences between the timbers themselves. Both are thermally modified using heat and steam without chemical preservatives, helping improve dimensional stability and durability.

Thermowood Pine is a Nordic softwood. It has visible knots and grain and offers a more natural, characterful appearance. It’s also the more cost-effective option. Thermo Ayous is a lightweight West African hardwood. It has very few knots and a smoother, more consistent appearance, making it particularly suited to contemporary projects. 

So your choice doesn’t necessarily start with the oil. It starts with the appearance you want from the timber, and Osmo 420 then enhances that appearance.

What Cladding Profiles Are Available?

The other thing to consider is the profile. Both Thermowood Pine and Thermo Ayous are available in several cladding profiles, so you can choose the timber and finish you prefer and then find a profile to suit the project.

Thermowood Pine profiles

Our Thermowood Pine cladding range includes:

• Thermowood Pine 20 x 140mm Channel Groove Vertical ST6.TW          
• Thermowood Pine 32 x 140mm 3-Stripe
• Thermowood Pine 20 x 95mm Channel Groove Vertical ST65.TW
• Thermory Thermowood Pine 20 x 115mm TGV C3 - Narrow Version
• Thermowood Pine 20 x 140mm TGV ST1.TW
• Thermory Thermowood Pine 20 x 140mm TGV C3
• Thermowood Pine 20 x 95mm TGV ST0.TW
• Thermowood Pine 20 x 140mm Channel Groove Horizontal ST10.TW

Thermo Ayous profiles

Our Thermo Ayous cladding range includes:

• Thermo Ayous 20 x 95mm Channel Groove ST65.TA
• Thermo Ayous 20 x 144mm Channel Groove Vertical ST6.TA
• Thermo Ayous 20 x 144mm 3-Stripe
• Thermo Ayous 20 x 144mm T.G.V. ST1.TA
• Thermo Ayous 20 x 95mm T.G.V. ST0.TA
• Thermo Ayous 20 x 70mm Parallelogram
• Thermo Ayous 20 x 144mm Channel Groove Horizontal ST10.TA

You can view the individual profiles online to compare the different finishes, available lengths and current prices.

Preparing Thermowood Pine and Thermo Ayous for Osmo 420

Whichever timber you choose, preparation is important. Freshly machined timber can have a polished surface known as mill glaze, which can prevent the oil from penetrating properly. Before applying Osmo 420, the boards can either be lightly sanded with 80-grit abrasive and cleaned down, or allowed to weather naturally for around six weeks.

Once prepared, apply two coats of Osmo 420, allowing approximately 10 – 12 hours drying time between coats. If the timber has already started to grey, it should be cleaned back before applying the oil. 

Do You Have to Oil Thermowood Pine or Thermo Ayous?

No. Neither timber needs Osmo 420 in order to be used as exterior cladding. If left untreated, both will naturally weather towards a silver-grey colour. The reason for applying Osmo 420 is if you want to help retain and protect the warmer appearance of the timber rather than allowing it to grey naturally. It’s therefore as much an aesthetic decision as anything else.

Which Finish Would You Choose?

This is where seeing the two boards side by side really helps. Watch our latest video to see Thermowood Pine and Thermo Ayous being treated side by side with Osmo 420 and compare the finished results for yourself. You can also browse our Thermowood Pine Cladding, Thermo Ayous Cladding and Osmo 420 UV-Protection Oil online, with live lengths and prices available on the Southgate Timber website. You can also order samples on our website, plus a sample sachet of the Osmo 420 to test the treatment yourself at home. 

FAQ’s

Can you use Osmo 420 on Thermowood Pine?

Yes. Osmo 420 can be used on properly prepared Thermowood Pine cladding. It deepens the colour and brings out more of the timber’s natural grain and knots.

Can you use Osmo 420 on Thermo Ayous?

Yes. On Thermo Ayous, Osmo 420 creates a warmer, richer colour while maintaining the timber’s clean and uniform appearance.

Does Thermowood Pine need treating?

Not for durability. It can be left untreated and allowed to weather naturally. Osmo 420 is an option if you want to help retain more of its original warm colour.

Does Thermo Ayous need treating?

No. Like Thermowood Pine, it can be left untreated and will gradually weather to a silvergrey colour. Apply a suitable UV oil if retaining the warmer colour is important to you.

What is the difference between Thermowood Pine and Thermo Ayous?

Thermowood Pine is a thermally modified Nordic softwood with visible knots and grain. Thermo Ayous is a lightweight thermally modified hardwood with very few knots and a cleaner, more uniform appearance.