Architects memo no. 104: September 2011
a stitch in time
You just know, when you feel that stab of pain in the tooth
that it isn’t going to go away by itself: and you know, joy of
joys that you will have to go to the dentist: and you know
that the sooner the better. But, procrastination rules, and,
by the time we make it to the dentist, what could have
been a relatively simple fix, has become more complex,
painful, time consuming and expensive.
Building maintenance, particularly in the area of coatings,
can also fall into a similar trap. All coatings, even the most
durable, are under constant duress. All exterior coatings are
attacked by U.V. light; rain (made slightly acid by dissolved
carbon di-oxide); heat; biological agents including moulds,
algae, lichen and, more or less, some physical abrasion.
Of course, there are many other possible aggressors such
as salt and other chemicals, but the above are ubiquitous.
Although acid rain can attack certain paint extender
pigments such as calcium carbonate, it is the organic
binder that is most susceptible to general attack. As this
binder is responsible for the adhesion of the coating; its
flexibility, cohesive strength and the gloss of the coating,
the degradation of this important ingredient is of concern.
This is, of course, why the paint industry puts so much
importance on the durability of its chosen binders.
Binder degradation occurs at, but not only at, the surface.
The first sign of degradation is a loss of gloss. As further
degradation occurs, pigment is left unbound on the surface
leading to the phenomenon of ‘chalking’. Concurrent with
this, however, some binder is being lost within the film. This
leads to an increase in pigment to binder ratio resulting in
a loss of cohesive strength and flexibility. In other words,
it becomes a poorer foundation for subsequent repaints.
At the onset of chalking, preparation for recoating is
typically a simple wash down. Where the coating is
chalking but otherwise sound, applying a clear coat
will significantly assist with durability. Loss of cohesive
strength is more difficult to quantify.
Testing is, by necessity, empirical. Your scribe’s favourite
test is to lightly scrape the surface with a sharp pen knife
blade. If the coating can be scraped off as a powder, then
the danger alarms should sound. Complete removal may
be needed or the use of a penetrating surface consolidator
may be indicated.
The onset of ‘chalking’ is a very good indicator that
maintenance should be indicated.
This has been written with relatively stable substrates in
mind – such as concrete.
When the coating is providing more than a decorative
effect over a stable substrate, other considerations come
into play. Untreated wood can rot and steel can corrode
when the coating system loses its ability to protect. Often,
the loss of protection, in these cases, may occur at isolated
points (rather than the general surface) and more critical
inspections are needed for such surfaces.
Corrosion on steel invariably occurs at spots where surface
preparation was inadequate or where coating film builds
were difficult to achieve, such as sharp edges. Once the
protection is lost at an edge, corrosion can travel, even
under the well protected areas. The saving grace of steel is
that degradation is almost immediately obvious, either via
rust staining or blistering.
Believe me, it will not stop. Maintenance should be
undertaken as soon as it appears and when the extent of
the damage is ascertained.
Timber is different and more difficult. It can quietly rot
underneath coatings that are substantially performing
well and it only needs a few inlets, such as unsealed end
grain, to open the whole substrate to attack. Without a
moisture meter it is difficult to know what is exactly going
on beneath a paint film, however, the presence of excess
water is invariably accompanied by an increase in the
surface grain profile.
The other area where premature failure occurs, as
mentioned above for steel, is over sharp edges, especially
on a ‘rusticated’ profile. Surface tension effects result in
substantially lower film builds over such areas and flaking
along the edge is common.
Uncoated timber degrades rapidly when exposed to the
weather, losing the binding lignins and hemi celluloses,
leaving a loose mat of cellulose fibres on the surface. The
longer flaked areas are left untreated the more labour
intensive the remediation becomes. All the degraded
surface must be removed in order to achieve a suitable
surface for recoating – a good guide being the restoration
of the original colour of the timber.
The message of this memo, and the facts, are clear that
prompt attention to paint degradation is the cheapest in
the long run.
Arghh! Ouch! That sore tooth again… nah! She’ll be
right! Yeah right!
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