When a garden structure requires a painted finish, the choice of timber is just as important as the colour. Paint reacts differently on various species, and in an outdoor setting where timber constantly shifts with changes in temperature and humidity, selecting the wrong type can result in a finish that cracks, peels, or fails within a few years. Accoya is one of the timbers that tackles this issue at its source, and it is worth understanding why.
What accoya actually is
Accoya is not a species in the traditional sense. It is a modified timber made from fast-growing, sustainably sourced radiata pine that has undergone a process called acetylation. Simply put, acetylation alters the wood at a cellular level by converting the free hydroxyl groups within the timber into acetyl groups. This process means the wood can no longer absorb moisture as easily as untreated pine. It does not swell, shrink, or warp with changes in humidity, and it becomes highly resistant to rot, decay, and insect attack.
The result is a timber that performs, in terms of durability and stability, significantly better than a conventional softwood, and in many aspects superior to traditional tropical hardwoods. It machines to a very smooth finish, which is important for painted joinery, and it has a genuine environmental credential: made from a fast-growing species through a non-toxic process, it is widely regarded as one of the most sustainable high-performance timbers currently available for exterior use.
Why dimensional stability matters so much for painted finishes
Paint on timber fails mainly because of movement. As timber swells and shrinks due to moisture and temperature fluctuations, it stresses the paint film. Micro-cracking occurs, water enters, and the paint starts to lift and peel. In species with poor dimensional stability, this can happen within a year or two of paint application, especially on south-facing surfaces that endure the full impact of sun and rain.
Accoya’s excellent dimensional stability greatly minimises this issue. Because the timber expands and contracts very little, the paint film is less exposed to cycle stresses. Coatings on accoya tend to last significantly longer between maintenance than those on conventional timber, and its smooth, uniform surface provides a better initial finish.
Accoya for painted garden joinery
At Stuart Garden Architecture, accoya is one of the timber species we use for painted finish garden structures, alongside sapele and redwood. It is especially suitable for applications where paint durability and longevity are important, such as pergolas and gates finished in a specific RAL colour, screening panels designed to match existing architecture, or structures in exposed locations where repainting may be challenging.
All our painted finishes are applied by spray in our own paint shop at the Somerset workshop, with one undercoat and two top coats as standard. This creates a smooth, even surface that would be difficult to achieve by brush, and enables us to match any RAL colour or specific shade from a leading paint manufacturer. The combination of a stable substrate and a properly applied finish provides painted garden structures with the best chance of performing as they should over the long term.
The sustainability argument
One of the less obvious benefits of accoya is its environmental profile. It is produced from radiata pine, a quick-growing species that can be harvested in just 25 years, compared to the hundred or more years needed by many traditional hardwoods. The acetylation process uses acetic anhydride, a compound related to acetic acid in vinegar, and produces no harmful by-products. The resulting timber is non-toxic and recyclable.
For clients seeking a painted finish on a garden structure without compromising environmental responsibility, accoya is a technically robust and sustainably sourced option worth considering.
Is accoya right for every project?
Accoya is well suited to painted exterior applications, but it is not the only option we work with, and it is not the right choice for natural finish structures. For natural finishes, we work with iroko, oak, cedar, and balau, each selected for their weathering qualities and outdoor performance. The appropriate timber for any project depends on the design, the application, the required finish, and the specific demands of the commission. We are always happy to discuss the options.
To discuss painted finish options for your garden structure, call us on 01984 667458 or contact us.