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Steel Designers' Manual - 6th Edition (2003)
1040 Corrosion and corrosion prevention
As the solvent evaporates, film-formation occurs, leaving the binder and pigments
on the surface as a dry film. The thickness of the dry film can be measured, usually
with a magnetic induction gauge.
The relationship between the applied wet film thickness and the final dry film
thickness (d.f.t.) is determined by the percentage volume solids of the paint, i.e.
d.f.t. = wet film thickness ¥ % vol. solids
In general the corrosion protection afforded by a paint film is directly proportional
to its dry film thickness.
35.6.2 Classification of paints
Since, in the broadest terms, a paint consists of a particular pigment, dispersed in a
particular binder, dissolved in a particular solvent, the number of generic types of
paint is limited. The most common methods of classifying paints are either by their
pigmentation or by their binder-type.
Primers for steel are usually classified according to the main corrosion-inhibitive
pigments used in their formulation, e.g. zinc phosphate, metallic zinc. Each of these
inhibitive pigments can be incorporated into a range of binder resins, e.g. zinc phosphate
alkyd primers, zinc phosphate epoxy primers, zinc phosphate acrylated-rubber
primers.
Intermediate coats and finishing coats are usually classified according to their
binders, e.g. vinyl finishes, urethane finishes.
35.6.3 Painting systems
Paints are usually applied one coat on top of another, each coat having a specific
function or purpose.
The primer is applied directly on to the cleaned steel surface. Its purpose is to
wet the surface and to provide good adhesion for subsequently applied coats.
Primers for steel surfaces are also usually required to provide corrosion inhibition.
The intermediate coats (or undercoats) are applied to build the total film thickness
of the system. This may involve the application of several coats.
The finishing coats provide the first-line defence against the environment and also
determine the final appearance in terms of gloss, colour, etc.
The various superimposed coats within a painting system have, of course, to be
compatible with one another. They may be all of the same generic type or may
be different, e.g. acrylated-rubber-based intermediate coats may be applied on to
an epoxy primer. However, as a first precaution, all paints within a system should
normally be obtained from the same manufacturer.