Architects memo no. 115: May 2014
a mine of information
by Colin Gooch, Resene Technical Director
Many years ago I was called by an alleged burglar, who was conducting his own defence, as an expert witness.
The Crown was claiming that a flake of paint, found
on a tool in the accused’s possession, was chemically
identical to paint on the burgled premises. The accused, quite rightly as it happened, argued that
many paints (at that time) were made to Government
specifications and it would be impossible to tell the
difference between two paints made by different
manufacturers, but to the same specification. What did
put him away, however, was not the chemical analysis
but the sequence of the layers of colours in the paint
flake that was identical to the system on the building.
The layers of paint that build up over the years of a
building’s life give insights not only of developing
technologies but also of changing fashion trends.
There are a couple of drivers for wanting to know about
what lies underneath the paint surface (apart from
apprehending felons), the most common being “will
the existing system be compatible with my proposed
repainting choice?” To be perfectly honest, there would
be fewer heartaches if this question was asked in a
timely manner - the reality for most is “The old paint
was perfectly sound until I applied the new paint – now
look at it!”
There was a long era in coating technology when, for
exterior paints, linseed oil reigned supreme; originally
pigmented with red lead for primers and white lead
for topcoats. Because of linseed oil’s high level of
unsaturation it has the remarkable ability to ‘dry’ when
exposed to air. Unfortunately, this hardening doesn’t
just stop when useful film properties are achieved but
carries on, slowly, over the years until the films become
extremely hard and brittle.
The next technological step was to use synthetic,
solventborne resins called alkyds. Initially these were
still based on linseed oil but the embrittlement was
somewhat reduced. Modern alkyds used for wood
primers are based on semi-drying vegetable oils (such
as soya oil) with the philosophy of trying to minimise the
level of unsaturation in order to achieve useful drying
times with a minimum of long term embrittlement.
100% acrylic primer binders contain no unsaturation
and their long term embrittlement is negligible.
The application of any new coat of paint induces some
degree of stress as there is invariably some contraction
moving from the liquid to the solid state. The stress can
be dramatically magnified if the repaint is accompanied
by a significant colour change. The move to a darker
colour, which will absorb heat from the sun in exposed
areas, can add intolerable stress to an aged and infirm
primer. While IR reflecting pigments will significantly
reduce the heat build-up of a system, success or failure
of the repaint will depend on how degraded the existing
In making the initial assessment of the existing paint
system, any sign of flaking should be taken as a sure
sign of primer degradation. Without any visible signs,
it is recommended to do a ‘tape adhesion test’, details
of which can be had from Resene. Failure of this test
indicates the need to completely remove the existing
system back to a bare substrate.
This is an expensive business but the ‘grit your teeth’
reality is that, eventually it will have to be done.
To compound the difficulty, the old system may be
comprised of some lead-based paint. This needs to be
tested for prior to stripping so that the appropriate
containment measures can be put in place.
There is another potential trap lurking underneath the
paint surface, an issue that was created between the
mid-sixties to the mid-seventies. This period saw the
advent in New Zealand of the gloss latex paint. These
paints were based on polyvinyl acetate co-polymers
with very low levels of pigment. The initial materials
proved to be extremely thermo- and hydroplastic, that
is, they softened significantly in the presence of either
heat or moisture and dramatically in the presence of
both. However, apart from some dirt pick up issues, they
worked reasonably well as finishes.
Problems can still occur now and then, where such
paints have been overcoated, particularly with darker
colours. If after prolonged rain such a system is rapidly
warmed in the sun, the softened gloss latex can be
literally ‘blown apart’ as the moisture within it vaporises.
A characteristic blister is formed in which both the top
and the bottom layer of the blister consists of the same
glossy paint; generally white and with a characteristic
topography. When this happens it is best to pour oneself
a stiff gin.
The other major reason for delving below the paint
surface is not so gloomy but is driven by the wish to
know how the building looked at the time that it was
Modern analytical methods are continually pushing
the limits of what can be determined by the chemical
constituents of various materials and artifacts, however,
often all that is needed to be known is the colour. Cross
sections of paint flakes do not present a large enough
sample for the colour to be precisely determined using
a spectrophotometer; however a good paint inspection
microscope in the hands of a good colour matcher will
get pretty darn close to what the original colour/concept
was meant to be.
I must admit to being somewhat of a heretic in this
area. Knowing something of the history and availability
of paints in our early days, colour scheme concepts
necessarily had to be restricted. What colour schemes
would our early designers have used if they had our
current palettes available? Probably nothing like what
they were compelled to use then!
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