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From the Resene paint and decorating problem solver

All coatings have a finite life and in some cases on an old house, be it 30–100 years old, the coatings build up, and although they can appear sound it is not until the surface is painted that the instability of the underlying coatings can become apparent.

Old enamel based paints including red lead primers become hard and brittle with ageing. Acrylic paints are more flexible than cross linking enamel paints and this means they can undergo extensions without film cracking. Standard acrylic paints are flexible but not elastic which means that at a certain level of extension the film will break. The extensibility is dependent upon resin features and gloss level. There are paints formulated using elastomeric acrylic resins that can undergo stretching and return to the non-extended state without film breakage.

As the thickness of paint builds up on a substrate there is a corresponding increase in tension on the underlying paint layers. Modern acrylic paints have excellent adhesion to clean, sound old prepared paints. When delamination occurs it is usually at the substrate primer interface. There are two distinct types of failure: adhesive (between layers of paint) and cohesive (within a given layer of paint).

Upon inspection of these issues the new coatings have always adhered well to the old coatings but the coatings underneath have lost all their adhesion and flexibility properties, causing them to pull away. When investigating a failure the best practice is to check what the failure type is and, if intercoat adhesion, which layers are involved. This can easily be seen by checking the back of a flake and comparing it to what remains where the flake delaminated from. If the failure is cohesive there is usually evidence of the same paint colour being present on the back of the flake and where the flake peeled from.

In most cases this issue can be reasonably unforeseen and does not even become an issue until the new coatings are applied, so the case of who is responsible becomes unclear. Even when adhesion tests are done on the old coatings, they still may appear sound but because there are so many layers (sometimes up to 300+ microns) they are brittle and their adhesion becomes limited. A common cause of delamination occurs with a change from a pale to a much darker topcoat colour. The extra heat associated with the colour change is the tipping point and delamination is common.

The issue can be avoided by identifying the age and build-up of existing coatings and fully removing them to a sound substrate suitable for painting. However, this does come at a much higher cost than simply painting over the old paint system and the risks involved with removing lead based paint add to the cost, so it is up to the homeowner or building owner to make an educated decision of which path they would like to take so as not to have any surprises when their newly painted home or building starts to blister with the failure of existing coatings.


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