By Scott Sidler
Boiled Linseed Oil (BLO) is a common item in my shop and in a lot of woodworker’s shops. It is a great oil treatment for woods, leaving a smooth touch on the surface. It revitalizes old dried wood and gives it a new life.
BLO is often mixed with other finishes and was once one of the main ingredients in most paints. There are still companies that make linseed oil paints like Allback if you’re interested.
Boiled Linseed Oil is not actually “boiled” like the name suggests, rather, it is chemically modified to encourage faster drying. Slow drying oils are a good thing, but regular linseed oil can take weeks or even months to fully cure in cold weather and that’s just too stinking long. Boiled Linseed Oil will dry in only a few days give or take depending on weather.
As awesome as this product is for both wood and metal, it has some dangers (specifically flammability) that need to be addressed in order to use it safely. Let’s talk about safety first.
Boiled Linseed Oil Safety
The hard truth is that Boiled Linseed Oil can spontaneously combust if stored or used improperly.
“Why on earth would you use it then?”
Well, gasoline is a lot more flammable than BLO and I don’t hear anyone calling their cars a death trap (except Corvair owners).
Here’s what happens: BLO 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 BLO create greater heat. Smaller quantities generate less heat.
How to Avoid Fires
- Always store BLO in metal containers. You can pour it into plastic containers for temporary use, but for long term storage it should be in a metal container.
- Any rags soaked with BLO should be laid flat on a non-flammable surface away from flammable items until they are completely dry or they can be placed in a metal container with water.
The most common source of BLO fires is from a wadded-up rag that has been soaked in BLO. 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.
Despite this issue, I still believe BLO is a great option for wood and metal. Read below, and you can learn how to safely use Boiled Linseed Oil.
Boiled Linseed Oil & Wood
BLO is a great protectant for wood both indoors and outdoors. It beautifies any wood and, once cured, protects the wood from sun and water damage. It can really reinvigorate old dried out wood and bring it back to a healthy status.
The powerful thing about an oil finish like BLO is its deep penetrating abilities. After application, the wood fibres draw the oil deep inside which protects not just the surface, but the whole piece of wood like in the image here.
Wipe on a couple coats of BLO on furniture, trim, or any bare wood and let it dry until it is no longer tacky (usually 24-72 hrs). The application is as simple as it gets and the results are more than worth the effort, which is why it has been used for hundreds of years by carpenters and refinishers.
Boiled Linseed Oil & Metal
A lesser-known use for BLO is to protect metal from oxidation. You can 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 BLO and you’re good to go.
Boiled Linseed Oil has one other thing that can cause concern, especially in humid climates. On exterior surfaces in humid wet 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. If you do get mildew, it’s not the end of the world as it can be easily cleaned off, but that’s why in Florida I prefer to use it mainly indoors.