Greying and maintenance of robinia

Ferdinand Terlinden

1 day ago

7 min read

Robinia series · Understanding the material

Patina, decking oil, wood reviver: what helps and what is pointless

A structure in robinia (Robinia pseudoacacia) — also known as black locust, and often sold as acacia — takes on a silver-grey patina within one to three years. That grey is a surface change: the layer altered by ultraviolet light is less than half a millimetre thick. Beneath it, the heartwood keeps its durability class 1–2 (EN 350) and its use class 4 (EN 335).

Keeping the honey colour is an aesthetic choice: a pigmented decking oil every two to three years, 4 to 5 litres for 20 m². Letting it go grey gives the same service life, around 30 years in ground contact.

less than 0.5 mm

The depth of wood altered by greying: photodegradation stays confined to between 100 and 500 micrometres.

Robinia pergola and decking naturally weathered to silver grey beside a swimming pool, no maintenance — A robinia pergola and deck left unfinished: the silver grey is a surface layer less than half a millimetre thick, not wear.

Why does robinia turn grey outdoors?

Greying comes from lignin — the substance that binds the fibres together and gives wood its warm colour. Ultraviolet light breaks it down at the surface, leaving only the paler cellulose and hemicellulose visible, and rain washes away the residues.

The change to grey is not direct, however, and it does not follow the same path in every part of the wood. Two things that are often confused need to be distinguished here: the sapwood, the pale, soft ring just beneath the bark, and the heartwood, the dark core that gives robinia all its durability.

Sapwood darkens first. On a half-round, a round stake or a rail that has kept its sapwood, the surface turns dark brown and then distinctly black in the first few months: extractives and residual sugars react to light and moisture, and surface micro-organisms settle in. That black then fades towards grey — but the softer sapwood cracks as it dries, and fine surface checks appear. This is an ageing of appearance, and it is also the part of the wood that does not last: that is precisely why we remove the sapwood on sanded posts and on all sawn products.

Heartwood, on the other hand, greys without going through black and without surface cracking. It moves from honey to silver grey in one to three years, the altered layer stays under half a millimetre, and the surface remains sound. That is what you see on a decking board, planed cladding or a sanded post, all with the sapwood removed.

In other words: if your half-round fence goes black and then cracks, it is not the robinia deteriorating, it is its sapwood doing its job as a sacrificial layer. And if your deck greys evenly without cracks, that is heartwood weathering exactly as expected.

The greyed layer is very thin: depth profiling by confocal Raman microscopy places photodegradation between 100 and 500 micrometres. All uncoated timbers grey in this way — teak, ipe, sweet chestnut and oak take on the same patina.

Does greying reduce robinia's durability?

Greying takes nothing away from robinia's durability: its resistance to fungi comes from flavonoids and tannins distributed throughout the heartwood, not from its colour. Nor does mechanical strength depend on the state of the surface — it lies in the cross-section of the piece, not in its outer few tenths of a millimetre.

A grey stake is not a tired stake. Timber at the end of its life shows itself in other ways: a screwdriver sinks in at ground level, the piece wobbles when you rock it, the section turns spongy. See “The real service life of robinia outdoors”.

Posts-and-rails fence in acacia half-rounds around a horse paddock, pale freshly installed timber — A posts-and-rails fence in robinia half-rounds around a paddock, on the day it was installed: the wood is still pale, sapwood included.

Blackened robinia half-round fence around a horse paddock, weathered sapwood before greying — The same kind of structure a few years later: the sapwood left on the surface has turned dark brown, almost black, and is starting to crack — while in the background an older fence has already turned grey. Two stages of the same patina; the cross-section itself is intact.

Why is the grey not uniform on the same structure?

Greying follows exposure, not the drawings. South- and west-facing surfaces turn in the first season; undersides, north faces and areas sheltered by an overhang stay honey-coloured for two to four years. Water running off an edge also leaves dark streaks.

This is customers' main disappointment, and it is better said before installation: the first two years are the most uneven. In the yard of our sawmill in Teiu, Hunedoara County (Romania), the stacks start to differ within a few weeks. The colour evens out from around the third year.

How do you keep the honey colour with decking oil?

A pigmented decking oil penetrates the pores without forming a film; its pigments act as a sunscreen. The surface must be clean, dry and restored with a wood reviver, then lightly sanded — keyed — with 80-grit paper. Apply in two coats, along the grain, between 10 and 25 °C.

Manufacturers' data sheets give 5 to 15 m² per litre per coat: robinia, being dense (around 750 kg/m³), sits at the top of that range — 10 m² per litre for the first application and 12 to 15 for maintenance coats, every two to three years. It is a lifelong commitment, and many people give up after two rounds — the wood then greys within a season.

Sanded honey-coloured robinia pergola against a stone house, new timber before greying — The honey colour of freshly sanded robinia: keeping it takes decking oil every two to three years, and changes nothing about the service life of the structure.

How do you restore and clean robinia that has already gone grey?

A wood reviver (de-greyer) is an acidic solution, usually based on oxalic acid — the acid found in sorrel and rhubarb — which dissolves the layer of degraded lignin. Apply it, leave it for 5 to 15 minutes without letting it dry, brush along the grain and rinse very thoroughly: allow 5 to 8 m² per litre, then 24 to 48 hours' drying. The result is a honey-amber colour, more matt than new wood; tannin stains and black rings from ordinary steel fixings — robinia is acidic, so stainless steel is essential — will not come out.

Routine cleaning is done with a soft brush, water and diluted soft soap, along the grain. A pressure washer must never be used at full pressure: the jet digs into the soft earlywood, leaves the latewood standing proud and makes the surface furry. Stay below 80 bar, with a flat nozzle at 40–60°, at 30 cm, and never use a rotary nozzle.

Which maintenance for which structure, and which products to avoid?

Film-forming stains and varnishes should be ruled out outdoors: the film cracks, peels off in patches and traps moisture. Raw linseed oil stays sticky, filters no ultraviolet and feeds mould: the wood goes black instead of grey. Fungicides and insecticides are unnecessary, as robinia meets use class 4 (EN 335) without chemical treatment.

The choice depends on how closely people come into contact with the wood. On a deck, furniture or a handrail, the wood is touched and seen close up: decking oil can be justified. On cladding or a pergola overhead, doing nothing is reasonable. On fences and stakes, the answer is clear: no maintenance, ever.

Structure	Maintenance worth doing	Realistic frequency
Decking, furniture, handrail	Gentle cleaning, then decking oil	Annual cleaning; decking oil every 2 to 3 years
Cladding, pergola, privacy screen	Let it go grey	Nothing, or an occasional wood-reviver treatment
Fences, stakes, cleft palings, posts	None	Never

Key points

· Robinia's greying is a patina less than half a millimetre deep: durability class 1–2 (EN 350) and use class 4 (EN 335) are unaffected.

· A grey stake is not a tired stake: end of life is judged with a screwdriver, not by colour.

· Sapwood and heartwood do not age in the same way: sapwood goes black and then grey as it cracks; heartwood greys without surface cracks.

· Grey arrives unevenly depending on the face and on water run-off; it takes three to five years to even out.

· Keeping the honey colour takes decking oil every 2 to 3 years, 4 to 5 litres for 20 m².

· Film-forming stains, varnish, raw linseed oil, fungicides and insecticides are pointless or harmful on robinia.

Frequently asked questions

Should a robinia deck that has already gone grey be sanded before oiling?

Full sanding is not necessary. On wood that has simply greyed, an oxalic-acid wood reviver, thorough rinsing and 24 to 48 hours' drying are enough to restore a sound surface. A light keying with 80-grit paper then improves the penetration of the oil into dense wood. Sanding through 40, 60 and then 80 grit is only justified if there is an old stain or varnish to remove.

Is greying uniform on every face of a structure?

No, and that is normal: greying depends on the dose of ultraviolet received. South- and west-facing surfaces turn in the first season, north faces and undersides keep the honey colour for two to four years, and an area sheltered under a wide roof overhang may never fully go grey. The appearance evens out between the third and fifth year on exposed parts.

Can a robinia deck be cleaned with a pressure washer?

Yes, as long as you never work at full pressure. Wood alternates soft zones, the earlywood, with hard zones, the latewood: too powerful a jet digs out the soft zones, leaves the hard ones standing proud and makes the surface furry, which is irreversible. Stay below 80 bar, with a flat nozzle at 40–60° rather than a rotary nozzle, at 30 cm.

Does decking oil stop the wood from splitting?

No. Decking oil slows moisture exchange at the surface and limits fine surface cracking, but it does not prevent surface checks — the cracks that appear when the outer layers dry faster than the core of the piece. Checks are a matter of wood shrinkage, not of finish, and they do not affect the performance of the structure.

Why has my robinia fence gone black instead of grey?

Because it has kept its sapwood. On round or split products — stakes, half-rounds, rails — the pale ring just under the bark stays on the surface: its sugars and extractives react to light and moisture, and the piece turns dark brown, sometimes black, within the first few months. It is temporary: this layer then fades towards grey, cracking slightly as it does. The heartwood beneath the sapwood is not affected and loses none of its durability. If you want a clean grey patina without cracking from the start, choose a product with the sapwood removed: a sanded post, decking, cladding or a sawn board.

How many litres of decking oil for 20 m² of decking, and how often should it be reapplied?

Allow 4 to 5 litres for 20 m² for the first application, in two coats: on a dense wood such as robinia, coverage is around 10 m² per litre per coat. For a single maintenance coat, 1.5 to 2 litres is enough, as coverage rises to 12–15 m². The useful interval is two to three years, reduced to one year on a fully south-facing site.