Architects memo no. 11: July 1981
the problem with life on paint films
Paint films not only have to resist energy attack, (infra-red
and ultra-violet radiation) and chemical attack from a host of industrial
and household chemicals, but also attack by a ubiquitous life form,
the moulds and fungi. On painted surfaces mould looks like dirt and
frequently the two cannot be visually distinguished. The most common
species are black but others can be brown or green. By wetting the surface
and rubbing, mould will show up as slime.
The theorist may be interested in knowing that moulds seldom live
alone on paint films but in a symbiotic relationship with enzyme producing
bacteria. Also that the substrate need not be the primary source of
nutrient - airborne dirt provides a perfectly satisfactory diet.
The practical man, however, should be aware that mould is a major
cause of paint disfigurement. It destroys the paint's fresh clean appearance
and if left unchecked shortens the life of the paint film. Unless removed
within a few months of its appearance, mould penetrates the body of
the existing film and once firmly established, will grow through subsequent
paint coatings, impairing adhesion and ruining the appearance of the
The practical man must also be aware that mould will only grow in
damp areas or where the relative humidity is greater than 70%. Most
high quality paints contain a fungicide but its killing ability is finite
and the mould will eventually take over. The only way to permanently
fix a mould problem is to remove the source of moisture and/or provide
Fungicide washes do however have a place in the scheme of things.
They can be used as a cosmetic treatment to clean up mould infested
surfaces where provision of adequate ventilation is not feasible. But
far more importantly they must be used to sterilise old surfaces before
It is particularly important to include such a treatment in the specification
where the surface to be coated is not smooth. Surface roughness, which
allows dirt to accumulate, will invariably harbour mould. Weathered,
unpainted concrete is just such a surface. (See Architects memo No. 7).
For years Resene marketed sodium salicyanilide as their Moss
& Mould Killer. This reliable material was one of the few non-toxic
fungicides available, and was available as a water-soluble powder. Difficulties
in manufacturing this material have caused one supply company after
another to pull out of this area until finally it is no longer available.
As a replacement for this material Resene has chosen to market sodium
hypochlorite solution under the trade name 'Resene
Moss & Mould Killer.' This material does everything that the
sodium salicylanilide does without the tendency to stain paint work.
The concentrated solution is diluted for use with about five parts of
water to one of concentrate. The water addition can be increased or
reduced depending on the severity of the mould infestation.
The product is available from Resene
ColorShops and stockists.
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