Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 145
the binding medium is essential (Wehlte,
1975). The ground is not only the foundation,
but the underlayer for all subsequent colour.
The visual character is determined by the binding
medium, whether the pigment particles are
simply attached or whether they are embedded
in it. The absorption of the medium by the
ground, and the distribution of the pigment in
the medium (opacity) are critical factors.
Finally the surface coating is important, not
only as a protective layer, but also in giving the
paint its optical properties.
Pigments are fine, solid, particles that do not
dissolve in the vehicle. They are used to provide
colour and body. Materials that provide
body without providing colour are called
extenders. Extenders are normally used
because they are cheaper than prime pigments
but they also contribute useful qualities to the
paint, for example by improving adhesion,
strength and ease of sanding.
A medium or binder is the substance that
holds the pigment particles together in a paint.
This must be capable when dry of forming a
solid coating in a continuous film over the surface.
This can be achieved by evaporation of
solvents, by reaction of pre-polymers, or by
change of temperature. The term medium may
also be used for materials suitable for adding
to or combining with paint to change its properties
to the requirements of the artists particular
techniques; for example to make it thicker
or thinner, quicker or slower drying. Additives
that may be used for this purpose include
thickeners, flow promoters and reducers, matting
agents, catalysts, accelerators, inhibitors,
wetting agents and dyestuffs. Binders for pigments
must satisfy most of the same criteria as
for a clear finish except that they need not be
transparent. Mixtures of pigments and binders,
or paints, can be categorized by the ratio of
pigment to binder. Paints with a high ratio of
binder to pigment are called fat and vehicular
and those with a low ratio are lean or granular.
This terminology may be extended to
lightly coloured mediums which are described
as pellicular.
The liquid part of the vehicle can consist
simply of a true solvent for the medium but
frequently it contains diluents which are not
true solvents for the binder but which contribute
other desirable properties to the
vehicle.
The formulation of paints is critical to their
success and many different factors play a part.
Paint must have the required appearance when
dry but must permit ease of handling to allow
application in the fluid state. All paints require
an appropriate degree of adhesion to the substrate
or preceding paint layer and good flow
characteristics to wet the surface and level out
to the required degree. They must then stop
flowing and dry to produce hard tough durable
films that will retain their decorative properties
in service. Generally, it is desirable that they
should be relatively easy to repair but they
must be resistant to common solvents in use.
Normally if a paint is being formulated for a
particular application one starts with the
required properties of the dry film. This determines
whether the film will be thermoplastic or
thermosetting and consequently the choice of
drying mechanism and application techniques
required. Formulation of paints in relation to
application characteristics is discussed by
Bentley (1998).
Pigmented varnishes are sometimes called
enamels, lacquers, finish coats or topcoats.
Enamels are normally based on thermosetting
materials that give a hard finish superficially
resembling vitreous enamel. The term lacquer
is normally used for thermoplastic solution
paints or varnishes but is sometimes confused
with oriental lacquer (e.g. Japanese urushi), a
quite different material. Although top coats can
be applied directly to the support, it is often
difficult to achieve the desired result with a single
formulation. Stoppers, fillers, primers and
undercoats which have a composition suited to
fulfil their function in the system of surface
decoration, are usually applied to form a
ground to give optimal results from the finish
layers. The function of stoppers and fillers has
been discussed above. Primers promote adhesion,
reduce absorption by porous surfaces and
usually impart some corrosion resistance over
metals. Undercoats, which frequently contain
significant quantities of extender, form a highly
pigmented layer that provides body to the
paint and help to level out minor imperfections
to permit easy smoothing and give a good base
for the topcoat. Undercoats must adhere well
to both the primer layer and top coat. Sealers
may be applied to the substrate over another
coating layer to prevent movement of material
out of the substrate into the paint or from one