Architects memo no.
88: September 2007
bread and butter
So how much paint do I really need? How far does it go? Is that
with or without thinning? Why did I end up with a litre leftover? Just
what is a spreading rate? Is spreading rate simply how far paint will
spread or is there some magical number?
Let's have a look at the current situation. A very common spreading
rate quoted by the industry is 16 sq metres per litre - but where
did this come from and how relevant is it? Looking for the source of
16 sq metres per litre will take you into the pre-metric days when the
standard spreading recommendation was 800 sq feet per gallon. This was
in the days of mainly enamel paints that had a volume solids of 50-60%.
Volume solids? This is the volume of goodies left in a paint can after
all of the solvents have evaporated off. Oil-based paints have invariably
had higher volume solids than their waterborne equivalents; which typically
pan out into the 35-45% range. An enamel paint of volume solids 55%
spread at 16 sq metres per litre will give a film that is about 35 microns
thick (1,000 microns equals one millimetre) and a three coat system
will give a total film thickness of about 100 microns. A waterborne
system spread at the same rate will only give about 25 microns a coat
and 75 microns for a three coat system (50 microns if a two coat, direct
to substrate, system is used).
Hundreds and hundreds of exposure panels have confirmed, beyond argument,
that thicker films offer more protection and the Resene philosophy has
been to quote a spreading rate that will result in a film thickness
of 35 microns per coat; 100 microns for a three coat system. This requires
cognisance to be taken of the volume solids of each paint.
This sort of regime is the absolute norm for steel protective coatings
where the film thickness of each part of the system is critically specified.
There is a property of a paint called the 'natural spreading rate'.
This rate is tied up with the application properties of the paint and
reflects how far a paint would be spread by someone simply painting
by 'feel'. The 'easier' a paint is to apply,
the further it will be spread and the thinner the resulting film will
be. A very easily applied paint may be able to be spread to 25 sq metres
per litre but the end result will not be satisfactory and result in
further coats having to be applied.
The paint formulator endeavours to build a viscosity profile into the
paint such that the specified spreading rate results no matter how facile
the painting process. However, changes from brush to roller, or the
addition of thinners or even tinters, can negate these efforts to precisely
control spreading rate.
So how about hiding power as a mechanism for film build control? Unfortunately,
different pigments have such wildly different hiding powers that it
is not a useful tool. A cheap fence paint, based on an equally cheap
natural iron oxide, may achieve fantastic obliteration at films so thin
that they do not provide sufficient protection from water ingress.
The best idea when painting is to estimate the amount of paint needed
based on the area being painted and the recommended spreading rate.
Then as the painting progresses keep an eye on what is being used. When
the midpoint of the job is reached, around half the paint should generally
have been used.
But what about interior, where appearance, rather than protection,
is the key? In these circumstances, it is correct that hiding is the
key rather than film thickness. The predictive issues here are: What
colour is to be used in the new coating and what colour is it going
An 'off-white' going over an 'off-white' may well produce an acceptable
finish in one, carefully, applied coat. A bright yellow going over a
dark purple feature wall may take four coats! So basically, while we
can bring technology to bear on exterior situations, where protection
is paramount, for interior, you may end up with a litre or so over.
But what the hell; you will not get a better colour match for 'touch
ups' and paint is not going to get any cheaper. It could be the best
investment you make!
Getting the spreading rate right on the wall is the bread and butter
of the painting business and the key to getting a quality finish. Get
it wrong and time and money will be wasted paying for more labour to
apply an extra coat just to get on the paint that should have been applied
in previous coats. Get it right and be surrounded by happy clients who
have enough money left to buy some jam.