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Architect's memo 108: August 2012

On the face of it, it would seem a reasonably straightforward issue – the painters have finished a new job; some alterations had to be made by another subbie, which damaged the paint work and now some touch-up work is required – so what is the drama?

Well… the devil is in the detail!

Let us set the scenario that the damage was to a paperfaced plasterboard interior cladding and that the repair was done using a gypsum-based pre-blended patching compound. We will also assume that the compound was skilfully applied, filling the hole flush and feathering out over the previously painted surrounding areas.

Being gypsum-based, and therefore hydraulically setting, the areas in the centre of the patch, being thicker, will have sufficient mixing water to fully cure while the feathered out areas over the existing paint will, almost certainly, be deficient in curing water and end up as a somewhat weak, friable layer.

This itself is not an unusual phenomenon which, to ensure good adhesion of subsequent coats, requires the application of a suitable penetrating sealer. If it were humanly possible to apply such a sealer only to the filler, then the complexities would be reduced; but this is impossible. Therefore the penetrating sealer must overlap onto the adjacent painted areas, which may not have any porosity at all.

Then comes the repaint which will find itself going over a) the original paint, b) the original paint with a sealer-rich surface and c) the sealed filler surface.

Putting aside the issues of getting absolute perfect batch-to-batch colour matching, if one applied one single brushload of paint over a small area of a surface that had already been coated with exactly the same batch of paint, it is almost certain that it would be noticeable – for two reasons. With waterborne paints, changes in the drying conditions can introduce very small topographical changes on the paint surface, leading to very subtle gloss changes. Further, the additional film thickness of the applied ‘patch’ will stand out as a slight plateau.

With the three different ’patch’ surfaces outlined above, variations in dry time and subsequent gloss variations are much more likely.

Automotive repair systems face the situation daily but, in this area, the repair is the primary function and not an afterthought. Further, the use of spray guns allows the very fine application of paint and a consequent ability to ‘blend’ the new with the old.

The brush and the roller simply do not have the same degree of sophistication as does the spray gun or airbrush.

Some decorative paints are easier to ‘touch-up’ than others. Paradoxically, these are generally also the cheaper paints! Paints that are extremely water-sensitive have the ability to re-dissolve the previously applied paint and so meld one coat with the other. Also, if one designs a ceiling paint, for example, with very little binder and lots of flatting agent, the paint is weak but very, very flat – regardless of drying conditions.

There is nothing more annoying than a beautifully painted, smooth surface marred by slight visible repaired areas. So what is the answer, other than a complete repaint? Well, in an ideal world, touch-ups should not be necessary. Of course this utopia will never be reached but we would go a long way towards it if we could get the planning right such that the painter really is the last trade on site!

Download as a pdf. (You will need Acrobat Reader).

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# 109 - another reprise – colour durability
paint and colour durability

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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