Linseed oil is a vegetable oil pressed from the seed of the flax plant. It is a drying oil: it does not harden by evaporation but by oxidation — the oil takes up oxygen from the air and forms a tough, water-repellent film.
That property has made linseed oil the binder of oil paint for centuries, and the most widely used way of saturating bare timber.
The three forms
- Raw (cold-pressed) linseed oil. Unprocessed and very thin. Penetrates deeply into timber, but dries slowly — days to weeks per coat. Suited to saturating timber rather than building a film.
- Boiled linseed oil. Heated and usually containing a drier (siccative), bringing drying time down to around a day. The usual choice for joinery. Note: because of the added driers, boiled linseed oil is not food safe; a worktop or a toy needs an oil expressly suited to that use.
- Stand oil. Linseed oil heated strongly in the absence of oxygen, leaving it thick and viscous. Forms a glossy, elastic and durable layer and yellows less. Used in traditional paints and as an addition to oil paint.
What you use it for
- Saturating timber. On garden timber, window joinery, sills and tool handles. The oil penetrates the wood fibre, slows moisture uptake and so reduces movement and checking. Unlike paint, linseed oil lets the timber breathe and does not flake — it wears away evenly and can therefore be refreshed without sanding or stripping.
- As a binder in paint. Traditional linseed oil paint grips exceptionally well on properly prepared timber and is known for its long service life on historic joinery.
- On concrete and stone. Linseed oil has been used to saturate concrete surfaces and reduce the uptake of water and salts; modified versions exist for that purpose.
- On iron. As temporary rust protection on cast iron and wrought ironwork.
Application
- Clean, dry timber. Grease, wax and old film-forming paint must come off; on a closed paint film the oil will not penetrate.
- Apply thinly. Several thin coats, each allowed to dry properly. A thick coat skins over and stays tacky underneath.
- Wipe off the excess. About twenty minutes after application, take off whatever has not soaked in with a cloth. What is left standing becomes a sticky, dust-catching film.
- Expect yellowing. Linseed oil yellows over time, especially in the dark. On pale, white-painted interior work that is a drawback; on timber it usually falls within expectations.
- Mould prone outdoors. A pure linseed oil coat outside can pick up black mould growth. Versions with a mould-inhibiting additive perform better on garden timber.
Important: the fire risk of rags
This is not a theoretical risk. Rags, sanding dust, brush cleaner and cloths with linseed oil on them can ignite spontaneously. Oxidation is a heat-releasing reaction; with the material piled up, the heat cannot escape and the temperature climbs until it catches. Workshops and homes burn down this way every year.
Always do this: lay used rags spread out flat outside on a non-combustible surface until they are fully hardened, or store them completely submerged in water in a sealable metal tin. Never balled up in a bin, never in a bucket with other waste.
Related terms
- Stand oil
- Oil paint
- Wood preservation
- Stain
- Primer
- Siccative
- Putty
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.