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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. |
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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. Most 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. e.g. Tung oil.
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.
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Types of Oil Finish |
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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) |
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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. |
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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. |
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