Architects memo no. 103: June 2011
Although, unlike wine, we are unlikely to have a
dedicated room for the storage and continuing wellbeing
of paint, we do have a habit of collecting and
‘hanging on’ to it – notwithstanding the excellent
return service that Resene Paintwise provides.
We all know that the bit of leftover paint in the can, the
full can that was surplus to requirements or the full can
that we simply never got round to applying, will come
in handy one day. The unknown is whether, when that
day comes round, the paint will still be fit for use or will
it be past its ‘use by’ date. How long does paint last in
The answer is “how long is a piece of string?” However
before you screw this up and aim it somewhere, here
are a few things to help you decide for yourself if it is
‘fit for purpose’.
For starters, you can forget about paint cans of premixed
two pack coatings and moisture-cured products.
Sorry to insult your intelligence but I needed to get it
out of the way! Also it is prudent not to try to rescue
product in badly corroded containers.
We’ll deal with solventborne enamels and varnishes
first as they are the simplest. One of the enemies here,
particularly in unopened cans, is gravity. The specific
gravity of a solventborne enamel binder solution is
quite low – about 0.9. The specific gravity of the lightest
organic pigment is about 1.4; extender pigments and
flatting agents 2-3 and for inorganic pigments such as
titanium dioxide, iron oxide etc it is 4-5. Even though
the paint chemist has tricks to help beat gravity, settling
is a problem.
Settling can vary from soft, easily re-incorporated to
dense and hard. Getting good re-incorporation of the
latter is the key to reinstating a usable paint. First
decant the supernatant liquid; break up the ‘cake’
in the bottom of the can with a broad knife and stir
vigorously, preferably with a drill mixer (with the tin
well secured) adding the liquid part back in very small
amounts. Do not add further liquid until a smooth
paste has been achieved with the existing blend. If a
smooth bit-free paste can be achieved, then the paint
should be usable.
Some enamel paints, particularly (but not solely confined
to) black, experience a loss of dry due to drier absorption
onto the pigment surface. Slow drying is only a nuisance,
not a calamity and still results in useful films (providing
it doesn’t get covered in insects and dust during the
prolonged drying period). Drying can be sped up by the
application of heat or by the judicious addition of small
amounts of ‘terebine’ driers.
Part cans are particularly vulnerable to ‘skinning’. All
enamel binders want to start curing on contact with
oxygen in the air. Enamels contain volatile ‘anti-skinning’
agents to prevent premature curing. These are more or
less lost during the application process. When a partfilled
can is stored, the binder will react with the air in the
void in the can and ‘skin’. Ironically the harder the skin
the better, as it is easily removed and, more importantly,
provides a good air barrier to the paint underneath.
(Skinning is naturally more severe when dried paint in
the rim of the can precludes the lid from fitting tightly.)
In such circumstances, if the skin can be cleanly removed
with a knife, after stirring the paint can be successfully
re-used. With some formulations a hard skin does not
form and the oxygen permeates the bulk of the paint (or
varnish) causing it to gel. Although tempting, such paints
should be jettisoned. I say tempting because there exists
the possibility of using large doses of strong solvents to
recover them. If paint cannot be recovered with 10% of
its recommended solvent then it is beyond redemption.
Waterborne systems are more complex but the outcome
is generally one of two problems.
The most significant problem is loss of colloidal stability,
which results in increasing viscosity either in localised
centres (leading to ‘bits’) or uniformly proceeding from
very thick liquids, to gels, to solid intractable masses.
There are many possible reactions leading to this sort of
instability and it is one of the tasks of the paint chemist
to prevent it by wise formulating. This instability may
be seen as an excess thickening in what appears to
be perfectly normal paint or a ‘phase separated’ paint,
which has a watery layer at the top and a thick layer
There is a simple rule of thumb! If the gelled material,
when rubbed out between the thumb and forefinger,
rubs out smoothly, the paint is recoverable. If however,
the gel balls up into rubbery particles, it is past it!
In the former case the product can be re-mixed using
the guide given above.
The second problem for waterborne paints is bacterial
attack. Bacteria will inhabit any watery environment
and paint is no exception. Without the presence of
preservative, paints would be extremely vulnerable. As
waterborne paints have become more and more benign
to humans, bacteria have also enjoyed the hospitality
offered making the role of the preservatives more
The acrylic polymer particles and the pigments are pretty
much immune to damage from bacteria but critical
parts of the formulations such as some thickeners and
many dispersants and stabilisers are prone. Paints, as
they leave the factory, should be stable for years in
unopened cans but once the can is open bacteria can
enter from the air, thinning water and from the surface
being painted. As the bacteria burden increases, the preservative becomes expended and loses its ability to
protect. For this reason, it is prudent to work from a
separate paint pot keeping the main container tightly
sealed at all times and, when finished (even though it
may go against the grain), discard the unused material
in the paint pot.
Mild infestations do not seriously impair the ability
of paint to perform but the inevitable odour, which
invariably accompanies bacterial growth, may persist.
Heavier infestations will generally result in phase
separation and gelled sediment, which is best
Although the above sounds somewhat daunting, I have
used paints of both types which have been over 20
Interestingly, with regards to bacteria resistance, the
keeping properties of paint are directly opposite to wine
in the fact that whites keep better than reds! It seems,
from years of observations, that the large quantities
of titanium dioxide in white paints have an inhibitory
effect on bacteria. The taste doesn’t improve though!
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