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prima facie

Architect's memo 83: May 2006

At first sight, the concept of having a single paint that acts as its own primer and topcoats is appealing. It makes specifying easier and certainly makes the logistics of the painting contractor simpler.

The more traditional 'systems' approach is predicated upon the belief that the idiosyncrasies of the substrate need to be addressed with whatever technology may be appropriate and that the burden of also meeting the requirements of a topcoat (colour, cleanability, durability etc) will detract from meeting the specific substrate needs.

There is one school of thought (your scribe's actually - but there may be like-minded technologists out there) that claims that the primer is pre-eminent and that the topcoat's role is merely to protect this primer. Some designers would disagree but it is indisputable that there will never be a cataclysmic failure of a paint system as long as the primer remains in sound condition.

Not all substrates are equally demanding. A fully cured, autoclaved fibre-cement sheet is a relatively undemanding surface and decorative topcoats are perfectly satisfactory when applied direct to the surface. Poured concrete, however, can present a variety of problems such as high alkalinity, undercured friable areas and highly glazed areas, which may demand more specialist pre-treatment.

To push the point further, no-one in their right mind would expect to protect mild steel in a marine environment simply with decorative topcoats.

Typically primers are a different colour to the topcoat and this gives a strong incentive to apply an adequate amount of topcoat in order to obliterate the primer colour. The increasing popularity of self-priming systems has been used as an excuse to simply drop the primer completely and move from a three to a two coat system.

It is axiomatic that if any coat is contributing to the hiding power of a system it must be interacting with light - visible and, invariably, the damaging ultra violet. The concept of the protected primer, discussed in the third paragraph, is clearly not happening. But, I hear you say, "is this not a cunning plan to get us to use more paint and thus boost your profits?!"

Not so! Numerous exposures, particularly over timber, clearly show that one achieves more than double the durability with a three coat system than with only two coats. Over the medium term, three coats actually decreases paint usage.

One learns never to say never and also to expect the unexpected when it comes to human ingenuity. However, surveying the current state of the art technology, we believe that better long term value is delivered by a well designed systems approach.

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Architects memos
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.

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