Architects memo no.
45: June 1985
the art of coarse
Unlike the skills of Leonardo, Michelangelo, and their ilk, the
art of the coarse painter is to transfer quickly and uniformly the contents
of a can of paint onto the surface intended for coating. To properly
train such a painter, requires an apprenticeship of 8,000 hours of mixed
practical and theoretical training. These people then become the experts
on paint application.
The paint manufacturer's interest is more peripheral and selfish,
requiring only that the method of application is suitable for their
product. There are some basic differences in the different methods of
application that are a source of many enquiries and this memo will
attempt to deal with these.
Brushing uses lateral shear forces to spread paint over the surface.
This sideways movement is very important because it is an action that
can displace surface contamination just prior to the wetting action
of the paint. It is also an action that brings the paint into the closest
contact with the surface, and for this reason it is often recommended
for the application of primers. It is felt that brushing well into the
surface can overcome deficiencies or holidays in surface preparation.
Whilst there are of course specific brushes for specific jobs, the
best brushes invariably contain a lot of long bristles. Bristles have
an excellent natural taper, as well as a flagged tip - excessive wear
removes this tip and significantly reduces the efficiency of the brush.
Rolling uses a compressive force perpendicular to the surface to press
the paint onto it, followed by a tearing action as the roller comes
away. This action can simply roll over surface contamination whilst
the tearing action causes roller marking, paint fly-off, and bubbling.
High shear is applied to the paint and good wetting of clean surfaces
is achieved. The pressing action is very good for coating pitted
surfaces like concrete blocks. Far faster spreading rates are achieved
by roller than by brush.
There are several different coverings used for rollers as well as
differing pile lengths of the same material. Whilst a good background
knowledge of where to use has been built up by the roller manufacturer,
it is worth noting that there are several new developments taking place
within the paint industry centred around new thickening systems. These
have a direct bearing on application properties (such as flow, leveling,
spattering, foaming) and existing knowledge regarding suitable roller
types may not always apply.
Pad brushes use technology from both brushing and rolling. Whilst
still relatively new in this country and the victim of reaction, it
is the author's view that these pad brushes do some jobs superbly well
combining excellent appearance with good speed and tidiness. Some greater
difficulties in cleaning appears to be the only drawback.
Spray painting deposits a fine mist of aerosol paint onto the surface.
The aerosol is produced by breaking up the paint with air jets (as in
air assisted spraying) or by using jets of highly pressurised paint
itself (airless spraying). With air assisted spraying large volumes
of air are used which tend to hit the surface first and then bounce
back from the surface. This air bounce back slows down the oncoming
paint droplets and even carries some of them away. This means that the
paint hits the surface with a relatively slow velocity, and hence low
energy. The air however, can assist in cleaning the surface prior
to paint hitting it.
Airless spraying does not have to contend with this cushion of air,
and the droplets hit the surface with much more velocity and energy,
thus increasing the wetting power. Airless spraying guns can
deliver very high rates to the nozzle and great expertise is required
to achieve uniform films.
Spraying techniques can allow operators to lay down layers of paint
wet on wet and, provided the paint has the required capabilities
of sag resistance and ability to dry in thick films, this is a perfectly
acceptable technique of efficiently applying paint. This two coats
in one method precludes the ability to monitor number of coats by
using slightly different colours for each coat, but in the hands of
a reputable applicator, it ought to reduce labour costs without reducing