Interest shown in our last memo and the comments it raised has prompted us to return to the subject and discuss a topic which obviously is of great interest. It concerns the overcoating of old alkyd systems with acrylics.
Memo No.28 finished with the conclusion that if a substrate was sound and the weather favourable, an acrylic will generally give better performance. This raises the question of when is a surface sound? Let us consider a house built of heart rimu, 50 years old. It has been maintained conscientiously over the years in white oil-based or alkyd paint and has never appeared to need a burn-off. The surface is now lightly chalking and needs a repaint. It appears as sound as a bell! The owners decide they are tired of white and wish to paint it in a fashionable dark olive green.
The points to consider are as follows:
Consider now the effects of these points on our example. The old embrittled system (which does still have the strength to hang on) is suddenly jolted out of its normal temperature cycle range of 0-25°C and subjected to 0-80°C. This in itself is sufficient to cause the original primer to let go but the new higher temperatures can also cause the mobilisation of oils and resins present in the timber which had previously been stable at lower temperature levels. These reins migrate to the timber surface and the more volatile oils try to force their way through the film. Great pressure is needed to try and get through the years of build-up and even if the oils succeed in getting through, they then come up against the strong, oil-resistant acrylic barrier.
Breakdown of the system ensues, but because the acrylic is so tough and adhesive it does not allow the system to fail in small flakes and blisters but rather holds it together into very large spectacular blisters.
It has been confirmed that had the house been painted with an alkyd of similar colour, breakdown would have occurred at about the same time but not in such a spectacular manner as with the acrylic.
The example taken above is an extreme example but it must be remembered that any colour, or type, of paint applied to an old system is going to put some extra stress on that system. Eventually there is going to be one coat, be it alkyd or acrylic, which will break the camel’s back. At that stage only total removal of the old paint system will suffice as surface preparation.
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.
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