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Oil and oil-varnish finishes

Unfortunately there is a great deal of mis-understanding and mis-information about varnish. The word has had a variety of meanings over time, and even today, it is used very imprecisely, especially on the containers of manufactured products. Do not rely on what the container calls the contents.

Some useful terminology 

  Varnish: is made by cooking one or more oils with certain natural or synthetic resins, which combine to then become a new substance. There 2 types of varnish; 

  • spirit varnish, made by dissolving natural resins like sandarac, mastic and shellac, in alcohol or turpentine to facilitate application; (French polish)
  • and oil Varnish, made by heating natural hard resins like copal or amber until they melt, then adding to a drying oil like linseed or tung oil to facilitate application.

  Today most varnishes are based on drying oils and use synthetic resins like phenolic or urethane. Generally sold as oil-based varnishes, applied with a brush, but by thinning to the consistency of milk, can become a wiping varnish. (Tru-oil)

  Wiping Varnish: Similar to oil varnishes, but thinner to facilitate application by a wiping pad. It cures by reacting with oxygen in the air, through a process called 'polymerisation'.

  Lacquer: another term that has been much abused and mis-used. It originated in the orient, as a deep, glossy finish, built up slowly in many, many layers. When replicated in the west it became known as 'Japanning'. Today lacquers refer to finishing products that dry by solvent evaporation. Mosty often they are designed to be sprayed, but can also be brushed or padded.

  Oil: is extracted from natural plant parts (notably seeds), fish and petroleum. It is usually mixed with other substances as a finish.

  Solvent: dissolves a cured finish, turning a solid into a liquid.

  Thinner: this just thins the liquid substance. It can not turn the solid back to a liquid.


Types of Oil Finish

  Examples Comments
Straight oil tung oil,

linseed oil (raw & boiled)

extracted from the nuts of the tung tree. One of the most water resistent oils once several coats are built up.
made from the seeds of the flax plant. The least effective oil finish.
Polymerised oil polymerised tung oil,

polymerised linseed oil

Perform like varnishes. Heated in an oxygen-free environment to about 500F, it cures faster and is much harder. (Tru-oil)
Varnish (incl. polyurethane) varnish thinned with 2 or 3 parts of mineral spirits Often sold as oil, this is WIPING VARNISH. They cure much faster than oil, much harder and more glossy.
Oil-varnish blend (incl. polyurethane)   Often called Danish Oil. This has characteristics of both. It is harder than oil because of the varnish, and can be built up to a better thickness because of the oil.


How to decide which oil is which! (Remember names and claims, are a marketing tool)

Clues Raw or boiled linseed Pure Tung Polymerised Linseed or Tung Thinned Varnish or Polyurethane Oil/Varnish mix
Labelled correctly? Y Y N N N
Distinctive smell? Y Y N N N
thin coat quickly gets tacky under hairdryer? N N Y Y N
Excess smeary even if warmed? Y Y N N Y
Cures flat with excess wiped off? N Y N N N
Cures satin after several coats with excess wiped off? Y N N N Y
Cures glossy after several coats with excess wiped off? N N Y Y N
Cures wrinkled when puddled on glass? Y Y Y* N Y*
Cures smooth puddled on glass? N N N Y N
* Usually wrinkles less than raw or boiled linseed or pure tung oil.