Architects memo no. 97: November 2009
get 'em off!
Everyone knows that it is a complete waste of time
painting over an unsound surface but there are
instances where picking what it is or is not sound is
sometimes not obvious.
The very application of a new two or three coat paint
system itself adds new stresses on a surface. These
new stresses can be the ‘straw that broke the camel’s
back’ causing catastrophic failure to a system which,
while appearing sound, had incipient weaknesses.
Although not totally confined to this area, older timber
systems, originally primed with oil or alkyd based
primers, typify this problem. The nature of poly-
unsaturated oils and their drying mechanisms are such
that continued long term reactions occur, which lead to
embrittlement and shrinkage. While this phenomenon
occurs with all the polyunsaturated drying oils, some
oils are more prone than others. Linseed oil is one of the worst and most of us are familiar with new plastic linseed oil putties aging into rock-hard materials
with no flexibility or extensibility.
There are hundreds and hundreds of timber structures
around the country which at some stage of their
decoration or re-decoration have had an oil or alkyd
primer included as part of their system. It may take
up to 50 years, but these will eventually embrittle and
weaken to the stage that they are no longer a viable
foundation for a new system
Burning off or stripping back to bare timber used to be
a typical specification or practice in the past. Because
of current costs, however, such a step is often only used
when it is obviously needed.
Nothing is more frustrating or wasteful than to fully
repaint a structure which appears soun only to find
that after a few weeks or months the added stress
of the new system has caused the tottering, failing
old primer to ‘give up the ghost’. Flaking is a totally
unacceptable way of paint failing and total removal of
the old paint now becomes obvious.
The cost of removal, which was hoped to be avoided,
now comes on top of the cost of the ‘failed’ new re-paint. People get angry! From the owner’s point of
view everything was fine until your paint/specification
was used, so the fault appears obvious.
It is clear that a full repaint is an expensive test
procedure to determine whether full paint removal is
warranted!
Most fine print (and sometimes even in bold) from
paint manufacturers does foresee this problem and
recommends testing the adhesion and cohesion of an
existing paint system using an adhesive tape. Typically
the recommendation is to thoroughly clean a target
area, press on an adhesive tape; pull sharply at 90° to
the surface and, if any paint is removed, total system
removal is recommended in that area.
Weaknesses of the test include degree of cleaning;
pressure applied to the back of the tape and quality of
the tape (your scribe has been sucked into buying some
cheap adhesive tapes that obviously benchmarked their
products against Post-it notes). The test still remains
useful however and can be refined by scoring the test
area, using a very sharp blade, with a very narrow V
shape. If the tape is applied, and pulled off, to include
the finest part of the V, travelling up the V, a clearer
result is obtained.
At the very least all the northern exposed aspects and
all window sills should be tested in this manner.
It cannot be said that these problems are totally
historical. While acrylic chemistry is generally not prone
to long term embrittlement, acrylics have not yet been
generally accepted for pre-priming of timber. Although
a lot of work is going on in this area, it predominantly
remains the domain of alkyds. Given also that these
are often designed for ease of use on the painting line
(rather than for long term flexibility) it would seem that
the ‘blow torch to the house fronts’ must be factored
into specifications for some years to come.
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