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The Ultimate Guide to Linseed Oil

linseed oil

Linseed oil is one of the oldest and most versatile finishes that has been used for centuries to protect and seal wood, metal, and a variety of other products.

Linseed oil is derived from the seeds of the flax plant. The oil is obtained by pressing the seeds to withdraw the oil. Linseed oil, also known as flaxseed oil, is a colorless to yellowish oil obtained from the dried, ripened seeds of the flax plant. The use of it as a wood finish dates back to ancient times, with evidence of its use found in ancient Egypt.

The widespread use of linseed oil as a paint binder became particularly prominent during the Renaissance, around the 15th century. This period marked significant advancements in painting techniques, where artists began to prefer linseed oil over other mediums like egg tempera.

Linseed oil paint created a superior durability, glossy finish, and an ability to dry more reliably and create more vivid and blended colors than other mediums of the time.

Its popularity wasn’t just limited to artistic painting, as it was the preferred medium for painting buildings. The dominance of linseed oil paint lasted until the early 20th century when it was replaced by newer solvent oil-based paints and then today with water-based paints.

Linseed oil paint still remains very popular with historic preservationists and restorers because of its ease of maintenance and authentic finish. You can learn more about modern linseed oil paints here.

Boiled vs Raw Linseed Oil

Linseed oil in its raw unadulterated form is sold as “raw” linseed oil. Raw linseed oil, like other raw oils, is not great for use as a finish because it just takes too long to dry, but for conditioning wood it can work great.

Due to its slow drying characteristics raw linseed oil can penetrate deeply into wood (see the picture below) bringing the much needed oils more deeply than other finishes.

linseed oil penetration
The deep penetration of raw linseed oil into the wood

So what is boiled linseed oil? Boiled linseed oil is not actually “boiled” like the name suggests, rather, it is chemically modified using metallic oil drying agents to encourage faster drying.

Slow drying oils are a good thing like I mentioned earlier, but raw linseed oil can take weeks or even months to fully cure in cold weather and that’s just too long for most people, my self included. Boiled linseed oil will dry in only a few days give or take depending on weather. This makes boiled linseed oil the preferred choice for use in finishes.

Historically, the process of boiling linseed oil with lead oxide or other metallic dryers which act as catalysts was common, helping the oil to harden more quickly upon exposure to air. Today the lead has been removed and the safer alternatives are used making it relative safe for use by anyone.

Boiled Linseed Oil Safety

The hard truth is that boiled linseed oil, like any drying oil, can spontaneously combust if stored or used improperly.

Here’s what happens: the oil cures by a chemical reaction with the surrounding oxygen in the air not by evaporation like water based finishes. This reaction generates heat like most chemical reactions. The heat generated can be intense in certain circumstances and can lead to spontaneous combustion.

Larger amounts of oil create greater heat. Smaller quantities generate less heat. The good news is that is relatively easy to prevent spontaneous combustion if you follow the steps below.

How to Avoid Fires

  • Store in Metal Containers – You can pour it into plastic containers to use while you are applying a finish, but for long term storage it should always be in a metal container
  • Spread out Rags – Any rags soaked with linseed oil should be laid flat on a non-flammable surface away from flammable items until they are completely dry
  • Place in a Metal Container with Water – Used rags can also be placed in a metal container with water to prevent spontaneous combustion
  • Keep Away From Flammable Items – Always keep rags and oil away from items like sawdust, papers, and other small, easily ignited items
  • Store in a Fire Cabinet – Store linseed oil containers in a metal fireproof cabinet when not in use

The most common source of fires is from a wadded up rag that has been soaked in linseed oil. The rag is wadded up and thrown in the trash with wood dust, newspapers or other kindling.

Since it is wadded up, it generates more heat because there is no air to pass over it and cool it. It then heats up to the flash point of the surrounding materials and the fire starts.

Watch this video to see how this happens:

Linseed Oil Uses

As a stand alone finish I lean on boiled linseed oil rather than raw linseed oil. Boiled linseed oil won’t give you a super hard and durable finish like polyurethane or varnish, but with enough coats, it will eventually build up a beautiful and protective finish. I’ve used it for years, as well as my own custom blends to beautify and protect furniture, tools, and leather with great results.

Linseed Oil on Wood

Boiled linseed oil gives a very “close to the wood” finish where you can really feel the wood instead of layers of plastic poly on top. The trick is to add multiple coats over the course of a week or so by wiping on a generous coat, letting it soak in for about five minutes, and then wiping off the excess. 

Give it about 24 hrs between coats, and build up between three and six coats depending on how thirsty the wood is.

Boiled linseed oil really brings out the deep rich color of the wood and accentuates the grain. Even less attractive woods look pretty decent after being oiled. Finish the project with a coat of wax for even more protection and you’ll have professional looking results.

Linseed Oil on Metal

This not just a wood finish as it can be used quite well to prevent rust on metal as well. Either boiled or raw varieties can simply be wiped onto metal surfaces to prevent rust from forming.

You should apply a thin coat to non-moving parts and once dry it will protect and beautify the surface. Thick coats can get gummy which is another reason you don’t want to use this in moving parts which get stuck together.

I use it mainly for chisels, screwdrivers, heirloom tools, block planes, and more specifically cast iron tops of my “Big-Boy” tools in the shop like the table saw, mortiser, and band saw.

Sand off the rust and polish the metal with some 0000 steel wool, then wipe on a thin coat of linseed oil and you’re good to go.

Linseed Oil on Leather

A often unexpected fact is that it can also be used effectively to reinvigorate and replenish leather products to keep them from drying out or cracking. It’s an excellent leather conditioner and, similar to metal, one light coat wiped on occasionally will keep leather looking beautiful.

Use as a Pre-Treatment

Even if you are planning a painted finish, linseed oil can be used a pre-treatment to revitalized old dried out wood and ensure a longer lasting paint job.

UV rays break down the fibers in wood, and after enough exposure, wood begins to turn grey from the sun’s effects. Paint and other coatings won’t adhere well to this grey wood, so it’s important to treat the wood before trying to paint again. Follow these few simple steps to revive that old wood and get it ready for some fresh paint.

1. Sand Lightly

Using something like 120-grit sandpaper tans make sure there isn’t any dirt or loose wood fibers lingering. Sanding also helps open up the pores of the wood to prepare it for the oil. Wipe off the dust, and you’re ready for oil.

2. Apply Oil

Mix up a 50/50 solution of boiled linseed oil and turpentine. Liberally apply the mixture to the wood using a cotton rag. You can brush it on if you’d like as well, but I prefer a rag. Make sure you apply a good amount to the surface, not just a light coat. When thinned with turpentine the oil will penetrate the wood and soak in quickly. If the surface is still shiny after a couple minutes, wipe off the excess and set your rag out to dry.

Let the oil dry for at least 24 hrs, but 48 hrs is even better. Don’t put a second coat on because with extra coats, the oil can build up on the surface and create adhesion problems with your paint. One coat is more than sufficient for use as a pre-treatment.

3. Prime & Paint

Using a good oil-based primer, cover the bare wood after the oil is cured. Using an oil-based primer rather than a latex primer helps create a bond between the boiled linseed oil and oil primer, which gives your paint job an extraordinary bond.

Latex primers are water based and don’t work as well with this system. You can finish with a latex finish paint on top of the oil-based primer, but stay away from a latex primer in this situation.

Linseed Oil and Mildew

There is one potential drawback that can cause concern, especially in humid climates and that is mildew. On exterior surfaces in hot and humid climates it has a tendency to mildew. So, before you go coat all your adirondack chairs, test an area for a while and see how it does.

viking linseed oil
Purified Raw Linseed Oil

If you do get mildew, it’s not the end of the world as mildew can be easily cleaned off, but it is something to be aware of.

There are modern “purified” versions of linseed oil that are available from companies like Viking and Allback that have had all or most of the proteins removed from the oils which provide better mildew resistance.

In my experience these are not perfect, but they are far better than the hardware store versions in both quality and mildew prevention so I would go this route if you feel linseed oil finishes are something you are interested in exploring. You can find a good selection of linseed oil finishes and supplies at Earth and Flax online.

Final Thoughts

Is linseed oil something right for you? That depends on a lot of factors. While it’s is not my primary method of finishing, I can tell you that for me I use both raw and boiled linseed oil for a wealth of options in my shop.

Restoring old houses I’m often needing it to treat dried out wood prior to painting, protect my tools, and for some sporadic furniture refinishing. I thin out my glazing putty with raw linseed oil when it’s dry and I’ve used it on my leather tool belt to bring it back to life after years on the job site.

The point is, linseed oil is a critical part of my career as a window restorer and historic preservationist. The question is where can you use it to add value and protection to your work or hobbies.

Whatever you decide just remember to use it safely and follow the protocols I’ve outlined above to protect yourself and those working around you. Now go explore linseed oil for yourself!

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