Impregnating wood: DIY versus the industrial process
The Wood Guide · Treatment
TreatmentImpregnating wood
The difference between brushing on a tin of preservative yourself and industrial impregnation is not the product but the depth. A few millimetres versus the entire sapwood layer.
- Under pressure, not with a brush
- Protects against fungi and insects
- No substitute for maintenance
What impregnation does — and what it doesn't
Impregnation brings a preservative into the wood, so that fungi and insects no longer find anything to feed on.
Untreated softwood is vulnerable outdoors. Spruce is naturally in durability class 4 and pine sapwood in class 5: in contact with moisture and soil they fail within a few years. Impregnation changes that, because the preservative penetrates into the fibre and makes the wood unattractive to rot fungi and wood-boring insects.
What impregnation does not do is make the wood waterproof or fix its colour for ever. Impregnated wood keeps absorbing and releasing water, keeps moving, and simply turns grey in the sun. If you want to prevent that, you still need to apply an oil or stain — a second, separate treatment.
The big difference lies in how deep the preservative gets. Brushing or rolling takes it a few tenths of a millimetre into the surface. With industrial pressure impregnation it is forced through the entire sapwood layer. Saw through such a beam and you can see the greenish zone running deep into the wood.
All KING tables and benches in spruce are pressure-treated before they are assembled. That means the sawn faces and the insides of the joints are protected too — exactly the places you can never reach with a brush.
- What it protects against
- rot fungi and wood-boring insects
- What it doesn't do
- make wood waterproof or stop it greying
- DIY
- brushing, rolling, spraying or dipping
- Industrial
- under pressure, into the sapwood layer
- Recognisable by
- a greenish to olive tinge
- Aftercare
- optional: oil or stain against greying
- Sawing it through?
- treat the end grain again
To be fair: impregnated does not mean maintenance-free. The wood still turns grey, can develop hairline cracks and collects green algae on the north side. Anyone expecting impregnation to keep a table new for ever will be disappointed — it only keeps it healthy.
How do you impregnate?
The colour, over time
The colour change says nothing about the protection — that lies deeper than the surface.
Cross-section schematic; the layer from brush and dip bath is drawn enlarged — in reality a few tenths of a millimetre.
Browse the KING picnic tables →Impregnate it yourself or buy it pre-treated
With a brush, roller or dip bath
Worthwhile for small jobs and for wood you cut to size yourself.
- Only works on the surface
- End grain absorbs the most — apply twice there
- Repeat every few years
- Cheap, and can be done straight away
Under pressure in an autoclave
The wood goes into a closed tank; a vacuum draws the air out, overpressure forces the preservative in.
- Penetrates into the sapwood layer
- Even across the whole board
- The joints are protected too
- Not something to do yourself, but available to buy
Then you expose untreated heartwood. Treat every sawn face with a suitable end-grain product, especially if the face points upwards or goes into the ground. Water always gets in through the end grain.
What happens in the pressure tank
- DryingThe wood must first be dry enough. If there is too much moisture in the cells, there is no room for the preservative.
- VacuumIn the closed tank the air is drawn out of the wood cells. That literally makes room.
- PressurisingThe tank fills with the preservative and the pressure rises. The preservative is forced into the emptied cells.
- Draining and fixingThe pressure is released and excess preservative drains back. The wood then has to rest so that the active substances fix in the fibre.
You can recognise the result by its colour: impregnated softwood has a greenish to olive-brown tinge that fades outdoors to honey brown in the first months and then to grey. That colour change says nothing about the protection — that lies deeper than the surface.
What is still up to you
Doing nothing is fine
Impregnated wood that you leave alone weathers to an even silver grey and stays structurally sound. For many people that is exactly the idea.
Keeping the colour
If you want to keep the warm tone, apply a wood oil — the first time usually after a few months outdoors, then every one to two years.
Cleaning
Remove green algae with a soft brush and water. No pressure washer at full power: it knocks out the soft fibres between the growth rings.
Painting over it
Possible, but rarely a lasting success: on outdoor furniture that moves and gets damp, an opaque coat will sooner or later come loose. Stain or oil follows the movement of the wood better.
Frequently asked questions about impregnating wood
What is the difference between impregnating and staining?
Impregnation protects from within against fungi and insects. Staining or oiling protects the surface against UV and moisture, and keeps the colour. They are two different things; one does not rule out the other.
Do I still need to treat impregnated wood?
Not for its lifespan. You do if you want to keep the honey-brown colour — otherwise the wood turns grey. A wood oil is the usual choice.
How long does impregnated wood last?
Above ground and with normal maintenance it lasts many years; in permanent ground contact the lifespan is shorter. The quality of the impregnation and the location together determine how long.
Why is impregnated wood greenish?
That comes from the copper compounds in the preservative. The colour fades outdoors to brown in the first months, and later to grey.
Can I paint over impregnated wood?
Technically yes, but we advise against it on outdoor furniture: the wood moves, and an opaque coat will crack and peel in time. An oil or stain moves with it.
Is impregnating it yourself worthwhile?
For small jobs and for sawn faces, certainly. As a replacement for industrial impregnation, no: with a brush you will never reach the depth a pressure tank achieves.
Prefer it impregnated from the start?
Our KING furniture in spruce comes pressure-treated from the factory — including the surfaces you would never reach yourself.
Have a question about your project? Just ask — we're happy to help.
The Wood Guide
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