the problem of life on paint films

Architect's memo 54: October 1998

Paint films have to put with an awful lot of abuse during their life times. Not only do they have to resist energy attack in the form of infra-red, visible and ultra violet radiation but also chemical attack from a host of industrial and household chemicals. Add to this, physical attack from wear and tear and one would think that that is sufficient abuse for the poor old film. But no - worse is to come.

Paints are also under attack from a ubiquitous life form - the moulds. Otherwise known as mildew, mould generally describes a mixture of fungi and algae, the spores of which are constantly in the air that surrounds us. 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. Further, that the substrate need not be the primary source of nutrient - airborne dirt provides a perfectly satisfactory diet.

The practical person, 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 paint work.

The practical person 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 fix a mould problem permanently is to remove the source of moisture and/or provide adequate ventilation.

Fungicidal 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 repainting.

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 typical of such a surface.

Fungicidal washes based on sodium hypochlorite (such as Resene Moss & Mould Killer) are extremely effective at killing mould with the added benefit of bleaching the moulds clean. It is however important to realise that they have no residual effect; any ongoing effect must be provided by the paint itself or further maintenance.

As stated earlier, most high quality paints contain fungicide at a level designed to cope with average exposure. It is accepted that there are some occasions and micro climates where additional defence may be needed. As an experiment Resene is introducing into its Resene ColorShops an acrylic paint compatible concentrated blend of fungicides and algicides under the brand 'MoulDefender'. This product is designed to be added, at point-of-sale, when a particular need for extra protection is identified.

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