Douglas fir is a popular species for outdoor projects like cladding, fencing and decking. It can also be used for joinery, feature trusses and structural work.
The wood contains chemical compounds (extractives) that protect it from decay and moisture, meaning it is naturally durable and dimensionally stable.
This means Douglas fir does not strictly need any treatment before use outdoors.
Like other popular softwood species such as western red cedar and Siberian larch, it can be left to weather naturally.
How long does Douglas fir last outside?
In a temperate climate, expect an untreated Douglas fir outdoor project to provide a substantial service life of at least 15 years (and often much longer for cladding) before needing replacement.
But whilst hardy when used externally, fir’s appearance will evolve over time. After around six months of exposure to moisture and UV, its natural light reddish-brown colours will transition to a silvery-grey.
This process, photodegradation, is purely superficial and does not indicate any decay or structural problems.
As a natural material, Douglas fir may showcase some minor surface checking or movement over time.
To retain the wood’s original warm tones and even further enhance its stability, you could consider applying a treatment to your Douglas fir.
The best exterior finishes for Douglas fir
These can be grouped into transparent, semi-transparent and solid-colour stains.
Note that we are not being paid to feature any of the products below. They are just what we find to be the best options on the market.
1. Transparent stains – for the grey look, with good protection
Colourless, transparent stains produce very minimal change in immediate appearance. This allows the wood to naturally turn grey over time, whilst providing protection against moisture-related issues, like black spots, splitting and cracking.
A product we'd recommend falling into this category is Owatrol H4 Wood. Completely clear, it will have no impact on the colour of your Douglas fir, allowing it to age gracefully and slowly achieve the on-trend, natural grey look.
The weathered, grayscale appearance that eventually results from these treatments is popular with outdoor enthusiasts wanting a versatile ‘back to nature’ feel for their project.
These types of products are also typically silicone free, so great for maximum environmental friendliness.
Water-repellent preservatives typically require recoating every couple of years. Check the product manufacturer’s guidelines on reapplication.
2. Semi-transparent stains – for strong protection that showcases natural beauty
As the name suggests, semi-transparent (or semi-solid) treatments are available in a range of tints and can be used to subtly alter the colour of the wood.
They provide a higher level of environmental protection and require less frequent reapplication. Look for a non-film forming product, meaning it won’t flake or peel.
Semi-transparent finishes can also help Douglas fir’s naturally striking grain pattern stand out, adding depth and character to the finished look.
Within this stain category, there are:
- Oil-based products: Such as Owatrol Textrol HES. With tints from virtually clear to charcoal, you’re able subtly customise the appearance of your Douglas fir project. By penetrating the wood, it provides deep, long-lasting protection and doesn’t form a film.
- Water-based products: Such as Owatrol Aquadecks. This also penetrates the wood to seal and protect it. You can choose from a number of different shades available (including honey, movingui, teak and even graphite grey). Water-based formulas are also a more environmentally friendly option, with lower VOC content than oil-based alternatives.
Reapplication times for these types of finishes typically range from 2-4 years, but be sure to check the manufacturer’s guidelines.
3. Solid-colour stains – for maximum protection and customisation
For the most protective finish (admittedly at the expense of sacrificing much of Douglas fir’s warm, characterful good looks), a solid, opaque finish might be up your street.
When you invest in such a naturally beautiful species, it’s worth noting that solid-colour finishes will obscure some of Douglas fir’s distinctive wild grain pattern.
That said, solid-colour finishes offer robust, long-lasting environmental protection – as well as the opportunity to completely take control of the colour of the wood.
Again, manufacturers like Owatrol produce a range of solid-colour stains. Some products in this range can still allow the wood's grain texture to show through to varying degrees.
The need for a maintenance coat differs depending on the product chosen and exposure to the elements, but typically 6 years is to be expected.
How do I finish Douglas fir?
We’d recommend treating your Douglas fir before installation, to ensure all hard-to-reach areas are completely sealed.
- Choose the right conditions: A mild, clear day is perfect. Absolutely avoid any rain, as well as baking hot direct sunlight. This can affect the coating and increase risk of failure.
- Make sure the wood is dry: Moisture is one of the most common causes of coating failure.
- Use a bristle brush: A large, soft bristle brush gives the most uniform finish. Rollers, sprayers and drips are faster, but less consistent.
- Consider a wood cleaner: A solvent-free cleaner (such as Owatrol Aquanett) applied beforehand will help prepare the surface, although this isn’t strictly necessary.
- If using a solid-colour stain, apply a primer coat before applying the topcoat. Where two topcoat applications are needed, allow the first to cure as per the manufacturer’s instructions (usually for around 3–4 hours).
- First coat: Work in the direction of the wood’s grain pattern with smooth, even strokes.
- Second coat: Once the first coat has cured for the time specified on the product instructions, add another.
- Some types of finish, like Owatrol Aquadecks, should be applied wet-on-wet and not allowed to completely dry between applications.
Restoring, treating and maintaining older Douglas fir
If your cladding, fencing or decking is still structurally sound, start with a gentle clean with warm, soapy water and a sponge or cloth on a clear, mild day to remove any dirt.
Stubborn or ground-in dirt can be removed by applying a wood cleaner and restorer, like Owatrol Net-Trol. This product rejuvenates the wood, bringing back its natural colour without affecting the wood’s fibres, preparing it for fresh treatment.
Once clean and dry, you can reapply your chosen maintenance coat – a transparent, semi-transparent or solid-colour finish.
Planning a project with Douglas fir?
We import, machine and supply a wide range of Douglas fir products, including machined products for cladding, fencing and decking.
We also offer sawn timber for high-class joinery, feature trusses and structural work.
We stock the top grade available from first growth logs. Slow-grown and virtually knot-free, this timber has a close grain and is a soft yellowish brown colour.
We also stock all of the woodcare finishes listed in this article.
Contact our team today to discuss your requirements, or visit us in-person at our showroom and shop near Ripon, North Yorkshire.
