The only wood finish older than linseed oil is probably animal grease rubbed on wooden objects. This stuff, linseed oil has been around for centuries and a ubiquitous wood finish in the nineteenth century. It could be easily purchased even on the frontier and it was also easy to grow, although tough on the soil and produces stalks for making flax or linen yarn and cloth and with little equipment the seeds crushed and pressed to produce the oil.
In the raw form it takes months to dry and it has been known for many centuries that it could be made to dry faster by adding certain mineral substances or heating to a boil would improve its drying characteristics and allow the ‘boiled’ linseed oil to dry in about 24 hours.
The boiled linseed oil we get today is chemically boiled as opposed to ‘kettle boiled’ but the results are the same, it dries much faster than raw linseed oil. Raw linseed oil can be exposed to sunlight and it will thicken and begin to polymerize (dry). The same can be done with boiled linseed oil and it produces what is called stand or sun thickened oil. Added to regular boiled linseed oil these cause the oil to flow better and dry faster.
I rarely use linseed oil straight unless I want a real fat oil, I generally thin it 50% with spirits of turpentine to make it a lean oil, to help in penetration and to add the subtle characteristics that it contributes to the final finish structure as well as to cause it to dry faster. I do not use other thinners like mineral spirits because it is a twentieth century invention, is dangerous, stinks, etc.
The trick in achieving a decent oil finish is to follow a few simple instructions. Put on an adequate coat of linseed oil to all surfaces, applying more oil where it absorbs in and rubbing it with your hand or a cloth in a way to cause friction, hence heat which will help drive in the oil and helps in drying. Then wait 10 minutes or so and wipe off all excess. This is very important that all excess is removed from all surfaces. The surface may be uneven, but don’t worry, this is just the first coat. Just make sure the surface doesn’t have a large amount of oil, as it will skin over and remain sticky.
Then depending upon the temperature and humidity, it will dry in 24 hours, longer at lower temperatures and higher humidity. An increase of 10° (F) in temperature will half the drying time and air circulation also helps in the setting or drying. After the surface is completely dry the surface can be scraped or sanded smooth, the dust removed and a second coat is added following the above directions. The second coat is usually a fatter oil (60% or more oil and 40% or less turpentine) which will adhere better following the old adage fat over lean.
Here is another traditional idiom:
One coat a day for a week, one coat a week for a month and one coat of month for a year.
Linseed oil as it dries forms a linoxyn film and with repeated application can produce a ‘film’ finish. A linoxyn film has no known solvents and must mechanically wear off. A few coats of linseed oil will produce a satin finish; repeated applications will improve the sheen with every properly applied finish.
It is a good idea to scrape or sand the dry finish between all coats except the final. If there is any roughness on the final finish then wiping off with a coarse linen cloth (there fibers are sharp) will produce a tacitly smooth finish.
To renew a worn oil finish, the surface should be washed with soap (not detergent) and dried completely, then the surface is lightly sanded, the dust removed and additional coat(s) applied as needed, again following the above directions.
It is an easy to use traditional finish that is made from natural renewable materials, you can grow and make your own oil if you want, or you can buy a gallon and start finishing with this excellent and proven finish. Linseed oil can also be a medium for stains and a solvent for some dyes; mixing the oil with dry powdered pigments or artists oil colors or with alkanet root for a fine wood dye.
Linseed oil is an excellent first coating that enhances the look of the wood by ‘popping’ the grain and can be over-coated with shellac or varnish and is also a main ingredient in oil varnishes. It can also be used as an isolating coat between coats of shellac.
And as always dispose of oily rags properly. They should be spread out flat in a dry airy place, preferably outdoors, stored in an air tight container, burned immediately or placed in water to prevent them from catching on fire and burning down your shop and house. A wad of oily rags will catch on fire by spontaneous combustion, so be careful.