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Douglas Fir Treatment Guide: The Best Sealants, Finishes & Stains & How To Apply Them

Douglas Fir performs superbly when used outdoors even without treatment. But with a little woodcare finish, you can be sure of decade upon decade of impeccable service and natural good looks.

Also known as Douglas Spruce and Oregon Pine, Douglas Fir is a fabulous species for any cladding, fencing and decking project. Its resins naturally repel those destructive forces it encounters and with an average dry weight of 540kg/m³, it defies its softwood status by providing handy resistance to bumps, knocks and abrasions.

Oh, and did we mention Douglas Fir’s stunning reddish-brown and blonde colour? Or its characterful, wild and alluring grain pattern?

If you have some of this gorgeous wood on your hands, you can’t be blamed for wanting to take care of it.

The best types of treatment for Douglas Fir for outdoor use

Thanks to its excellent natural durability, Douglas Fir can be left to weather naturally. However, moisture, humidity and the sun’s UV rays will eventually cause your timber to begin changing colour to a silvery-grey after around six months. After this, you may begin to see some level of surface mould build up, with potential for black spots.

This can be offset with a quality, non-film forming treatment. You have many options when finishing, sealing or staining Douglas Fir for outdoor use, which we would group into three categories depending on their appearance and the level of protection provided: transparent, semi-solid or a solid finish.

Option 1: Transparent preservatives — to achieve a natural look whilst providing good protection

This is the least ‘intrusive’ type of treatment. Colourless, transparent preservatives result in very minimal change in immediate appearance, and allow the wood to naturally turn grey over time whilst providing decent protection against damage caused by moisture, bacteria and dirt — avoiding those dreaded black spots!

A good product falling into this category is Owatrol H4 Wood. It provides colourless, waterproof protection, allowing your Douglas Fir to eventually achieve the on-trend, rustic ‘grey look’ whilst guarding against the structural problems associated with leaving the wood completely untreated — we’re talking cracking and warping.

These types of products are usually silicone free, so fantastic if you’re wanting to create a natural, sustainable space. As people increasingly seek to get ‘back to nature’ with their gardens, the weathered appearance that comes with this type of treatment can eventually provide a versatile, uncomplicated monochromatic backdrop to any outdoor area.

Water-repellent preservatives don’t usually last as long as the stronger types of finish which we’ll mention later — they typically require a recoating every couple of years. Be sure to check the product manufacturer’s guidelines on reapplication.

P.S. — don’t fancy waiting for your wood to turn grey? Want that rustic look for your Fir as soon as possible? You might like to look at a weathering accelerant instead. Si00x is a popular choice for speeding the greying process whilst sealing and protecting the timber.

Option 2: Semi-transparent stains— for strong protection that preserves & showcases the Douglas Fir’s beauty

The second category of finishes for Douglas Fir are semi-solid or semi-transparent oils. These provide a higher level of environmental protection for the wood and require less frequent reapplication, and they can be used to subtly alter the colour of the wood.

A quality semi-solid finish can actually enhance the Douglas Fir’s natural, wild and characterful grain pattern, helping it to ‘pop’, which is why we’re big fans of these types of finish.

When shopping around for a quality stain for your Douglas Fir, be sure that the product is non-film forming. This means it won’t flake or peel.

Within the semi-transparent stain category, there are oil-based and water-based products.

A popular oil-based semi-transparent finish includes Owatrol Textrol HES. Available in a range of tints, from virtually clear to charcoal, you’re able to add a degree of customisation to the appearance of your Douglas Fir project. By penetrating the wood, it provides deep, long-lasting protection and doesn’t form a film.

As for water-based semi-transparent finishes, you’ve got Owatrol Aquadecks. This similarly penetrates the wood to seal and protect it without forming a film. You can choose from a number of different shades available (including honey, movingui, teak and even graphite grey). As it’s water-based, it is seen as more environmentally friendly.

Reapplication times for these types of finishes typically range from 2–4 years, but be sure to check the manufacturer’s guidelines.

Douglas Fir cladding being stained with a brush.

Option 3: Solid-colour finishes— for maximum protection & customisation

If you don’t mind sacrificing much of Douglas Fir’s warm, characterful good looks and you’re seeking the most protective type of finish with the lowest requirement for reapplication, then a solid, opaque finish might be up your street.

Now we’ve got to admit that we’re not huge fans of these types of treatments, especially on a species as naturally beautiful as Douglas Fir, when much of its appeal is the result of wild grain.

That said, solid-colour finishes offer robust, long-lasting environmental protection — as well as the opportunity to completely take control of the colour and appearance of the wood, perhaps as part of a bold design.

Owatrol produces a wide range of solid-colour stains that can be used to customise the colour of your cladding. Some types of solid-colour finish can still allow you to maintain the wood’s original grain pattern to an extent.

The need for a maintenance coat differs depending on the product chosen and exposure to the elements, but you’re usually safe for at least 6 years.

How do I apply a finish or treatment to my Douglas Fir cladding, fencing or decking?

Before applying any finishes to new (not old) Douglas Fir, you might like to apply some Owatrol Aquanett to prepare the Fir for treatment, although this isn’t strictly necessary.

Treatments are usually best applied with a quality brush on a mild, clear day, but you can achieve decent results with a roller, pad, spray or drip. Using a brush almost always takes longer, but we’d say it’s worth the time investment to guarantee an even, smooth application.

Make sure the wood is initially dry when applying your chosen finish. You might want to add another layer after a certain amount of time specified by the product instructions (be aware that many types of finish, like Owatrol Aquadecks, are intended to be applied wet-on-wet and should not be allowed to completely dry between applications).

The treatment process can differ for solid-colour stains. Prepare the wood with a primer coat before applying the topcoat. If the wood requires two topcoat applications, be sure to allow the initial topcoat to cure as per the manufacturer’s instructions; this can be 3–4 hours.

If you’re treating a new Douglas Fir deck, cladding or fencing project, we’d recommend waiting around 2–3 months after installation before applying the finish — or you can just let it weather naturally!

What about restoring, treating and maintaining older Douglas Fir?

A few years after installation and treatment, you might notice your wood’s treatment wearing slightly thin. If your cladding, fencing or decking is still in good structural condition, give it a gentle clean with warm, soapy water and a sponge or cloth on a clear, mild day to remove any dirt.

Once clean and dry, be sure that any existing finishes and oils are cleared — a quick treatment of Owatrol Net-Trol can brighten, restore and neutralise the wood, preparing it for fresh treatment. Then, you can reapply your chosen maintenance coat — a transparent, semi-transparent or solid-colour finish as per the manufacturer’s guidelines.

How long will Douglas Fir last outside?

This depends on how heavily exposed to the elements your wood is — whether it’s at an elevation, whether it’s south facing and even whether you live near the sea! However, as a rule, with treatment and a little TLC, Douglas Fir usually provides around 35 years of service.

Felled Douglas Fir timber.

Got a project in mind?

We’re here to help. As well as stocking a wide range of Owatrol woodcare finishes, we offer cladding, fencing and decking in a range of beautiful, durable timber species — which, of course, includes Douglas Fir! All of our timber cladding is machined to profile on site from kiln dried stock.

Whether you’re an architect, specifier or home improver, our team pride themselves on providing top-notch timber products with first-rate customer service. From your initial enquiry through to the delivery of goods (and all those queries, questions and pain points in between), your project handler is here to help.

To get started, drop us a message using the button below. Alternatively, you can reach us through our website contact page, by phoning 01765 640 564 or by emailing

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