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Steel Designers' Manual - 6th Edition (2003)
1044 Corrosion and corrosion prevention
35.7 Application of paints
35.7.1 Methods of application
The method of application and the conditions under which paints are applied have
a significant effect on the quality and durability of the coating.
The standard methods used for applying paints to structural steelwork are brush,
roller, conventional air-spray, and airless spray, although other methods, e.g. dipping,
can be used.
(1) Brush. This is the simplest and also the slowest and therefore most expensive
method. Nevertheless it has certain advantages over the other methods, e.g.
better wetting of the surface; can be used in restricted spaces; useful for small
areas; less wastage and less contamination of surroundings.
(2) Roller. This process is much quicker than brushing and is useful for large flat
areas but demands suitable rheological properties of the paint.
(3) Air-spray. The paint is atomized at the gun-nozzle by jets of compressed air;
application rates are quicker than for brushing or rolling; paint wastage by overspray
is high.
(4) Airless spray. The paint is atomized at the gun-nozzle by very high hydraulic
pressures; application rates are higher than for air-spray and overspray wastage
is greatly reduced.
Airless spraying has become the most commonly used method of applying paint
coatings to structural steelwork under controlled shop-conditions. Brush and roller
application are more commonly used for site application, though spraying methods
are also used.
35.7.2 Conditions for application
The principal conditions which affect the application of paint coatings are temperature
and humidity. These can be more easily controlled under shop conditions than
on site.
(1) Temperature.Air temperature and steel temperature affect solvent evaporation,
brushing and spraying properties, drying and curing times, pot-life of
two-pack materials, etc. Heating, if required, should only be by indirect
methods.
(2) Humidity. Paints should not be applied when there is condensation present on
the steel surface or the relative humidity of the atmosphere is such that it will
affect the application or drying of the coating. Normal practice is to measure
the steel temperature with a contact thermometer and to ensure that it is maintained
at at least 3°C above dew-point.