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Prior to this discussion on primers, four factors which were
assumed to be perfect before painting were mentioned. It may be opportune
to discuss the consequences if these factors are less than perfect.
- Pretreatment.
The two major pretreatments used in New Zealand are boron and copper-chrome-arsenate,
used to greater and lesser degrees. It is important to realise that
the latter treatment is a fixed salt and the boron a soluble salt.
Boron treatment should only be used in dry areas, unless protected
by a substantial, durable paint system. Highly permeable systems or
systems prone to checking can allow the soluble salts to be leached
from the timber, thus increasing the risk of the timber rotting.
- Moisture content.
The moisture content of timber at the time of painting should ideally
be as close to the moisture content expected in service. Too high
a moisture content can lead to blistering and flacking problems, too
low a level can lead to cracking following the swelling of the timber
when it comes up to it's equilibrium level. Moisture problems occur
less frequently with waterborne paints than with other systems.
- Storage.
Incorrect storage can result in the contamination of the timber with
a variety of contaminants that can affect paint performance, i.e.
mould, soil, fuel oils, cement dust etc. Individual discussion of
these points is not necessary as the problems are self-evident. A
less easily distinguished problem is the effect of weathering which
can leave a loose mat of cellulose fibres on the surface or lead to
checking of the timber surface. Unless weathered timber is sanded
back to a sound surface, premature failure of the paint, usually by
flaking, is certain.
- Detail and design.
It is a phenomenon of all liquids that, due to surface tension effects,
they will pull away from sharp edges resulting in a decrease in thickness
at those edges. In paint systems this can result in sharp edges being
only half as thick as the rest of the paint system. As sharp edges
also tend to become drip points for water, they can easily be identified
as weak spots in the system. Practice bears this out as failure of
paint on exterior woodwork that invariably starts as cracking then
curling-back from a sharp edge. The radiusing of sharp edges even
by as little as a 5mm radius, can increase paint durability by a more
significant amount than any of the formulating principles known to
the industry at present.
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