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VARNISHES top - dust is a problem with all varnishes, specks of dust can ruin
the look of an otherwise perfect finish. For a durable finish, always apply a
thinned first coat so that it will 'key' into the timber.
Acrylic Varnish
Quick drying and non-toxic, brushes can be rinsed out in water. Available in
gloss, satin and matt finishes.
Exterior
Specially formulated microporous varnish, which allows the wood to 'breathe'.
Also, contains an ultra violet filter to reduce bleaching by the sun, and a
fungicide to prevent mould growth.
Polyurethane
Gives a clear, tough surface, and is available in gloss, satin and matt
finishes. Once fully hardened, they should not be marked by hot cups etc. being
rested on them.
Yacht
A high gloss finish based on tung oil and phenolic resins, with extra durability
and, as the name suggests, resistant to sea, fresh water and all climatic
conditions. Very good for use on outside timber even when well away from the
sea.
Varnish is a transparent, hard, protective finish or film primarily used in wood
finishing but also for other materials. Varnish is traditionally a combination
of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually
glossy but may be designed to produce satin or semi-gloss sheens by the addition
of "flatting" agents. Varnish has little or no color, is transparent, and has no
added pigment, as opposed to paints or wood stains, which contain pigment and
generally range from opaque to translucent. Varnishes are also applied over wood
stains as a final step to achieve a film for gloss and protection. Some products
are marketed as a combined stain and varnish.
After being applied, the film-forming substances in varnishes either harden
directly, as soon as the solvent has fully evaporated, or harden after
evaporation of the solvent through certain curing processes, primarily chemical
reaction between oils and oxygen from the air (autoxidation) and chemical
reactions between components of the varnish. Resin varnishes "dry" by
evaporation of the solvent and harden almost immediately upon drying. Acrylic
and waterborne varnishes "dry" upon evaporation of the water but experience an
extended curing period. Oil, polyurethane, and epoxy varnishes remain liquid
even after evaporation of the solvent but quickly begin to cure, undergoing
successive stages from liquid or syrupy, to tacky or sticky, to dry gummy, to
"dry to the touch", to hard. Environmental factors such as heat and humidity
play a very large role in the drying and curing times of varnishes. In classic
varnish the cure rate depends on the type of oil used and, to some extent, on
the ratio of oil to resin. The drying and curing time of all varnishes may be
sped up by exposure to an energy source such as sunlight or heat. Other than
acrylic and waterborne types, all varnishes are highly flammable in their liquid
state due to the presence of flammable solvents and oils. All drying oils,
certain alkyds, and many single-component polyurethanes produce heat during the
curing process. Therefore, oil-soaked rags and paper can literally burst into
flame hours after application if they are bunched or piled together, or, for
example, placed in a container where the heat cannot dissipate. Spread rags and
paper out to dry. As an added safety measure, douse the waste materials with
water prior to drying. Alternatively they can be submersed in water in an
airtight metal container and kept for later use or disposal. Check with
manufacturer for details.
Polyurethane
Polyurethane varnishes are typically hard, abrasion-resistant, and durable
coatings. They are popular for hardwood floors but are considered by some to be
difficult or unsuitable for finishing furniture or other detailed pieces.
Polyurethanes are comparable in hardness to certain alkyds but generally form a
tougher film. Compared to simple oil or shellac varnishes, polyurethane varnish
forms a harder, decidedly tougher and more waterproof film. However, a thick
film of ordinary polyurethane may de-laminate if subjected to heat or shock,
fracturing the film and leaving white patches. This tendency increases with long
exposure to sunlight or when it is applied over soft woods like pine. This is
also in part due to polyurethane's lesser penetration into the wood. Various
priming techniques are employed to overcome this problem, including the use of
certain oil varnishes, specified "dewaxed" shellac, clear penetrating epoxy
sealer, or "oil-modified" polyurethane designed for the purpose. Polyurethane
varnish may also lack the "hand-rubbed" lustre of drying oils such as linseed or
tung oil; in contrast, however, it is capable of a much faster and higher
"build" of film, accomplishing in two coats what may require multiple
applications of oil. Polyurethane may also be applied over a straight oil
finish, but because of the relatively slow curing time of oils, the emission of
certain chemical byproducts, and the need for exposure to oxygen from the air,
care must be taken that the oils are sufficiently cured to accept the
polyurethane.
Unlike drying oils and alkyds which cure, after evaporation of the solvent, upon
reaction with oxygen from the air, true polyurethane coatings cure after
evaporation of the solvent by a variety of reactions of chemicals within the
original mix, or by reaction with moisture from the air. Certain polyurethane
products are "hybrids" and combine different aspects of their parent components.
"Oil-modified" polyurethanes, whether water-borne or solvent-borne, are
currently the most widely used wood floor finishes.
Exterior use of polyurethane varnish may be problematic due to its heightened
susceptibility to deterioration through ultra-violet light exposure. It must be
noted, however, that all clear or transluscent varnishes, and indeed all
film-polymer coatings (e.g. paint, stain, epoxy, synthetic plastic, etc.) are
susceptible to this damage in varying degrees. Pigments in paints and stains
protect against UV damage. UV-absorbers are added to polyurethane and other
varnishes (e.g. spar varnish) to work against UV damage but are decreasingly
effective over the course of 2-4 years, depending on the quantity and quality of
UV-absorbers added as well as the severity and duration of sun exposure. Water
exposure, humidity, temperature extremes, and other environmental factors affect
all finishes. By contrast, wooden items retrieved from the Egyptian pyramids
have a remarkably new and fresh appearance after 4000 years of storage. Even
there, however, fungal colonies were present, and mildew and fungus are another
category of entities which attack varnish. In other words, the only coat of
varnish with near perfect durability is the one stored in a vacuum, in darkness,
at a low and unvarying temperature. Otherwise, care and upkeep are required.
varnish
1. A viscid liquid, consisting of a solution of resinous matter in an oil or a
volatile liquid, laid on work with a brush, or otherwise. When applied the
varnish soon dries, either by evaporation or chemical action, and the resinous
part forms thus a smooth, hard surface, with a beautiful gloss, capable of
resisting, to a greater or less degree, the influences of air and moisture.
According to the sorts of solvents employed, the ordinary kinds of varnish are
divided into three classes: spirit, turpentine, and oil varnishes.
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