Cohesive failure is described as splitting of a primed surface and roof paint where the primer is stuck to both the underside of the topcoat and the substrate. Several factors can lead to cohesive failure in primed and painted roofs.
Cohesive failure or “splitting” is not common – it indicates that the adhesion of the primer is stronger than the film itself and results in the top coat, on the roof, peeling away from the substrate. This occurs when paint is sprayed while the temperature of the surface is too hot to paint (above 35C) and is applied too thickly.
The most likely cause of cohesive failure is:
When the substrate or surface to be primed and painted was too hot and paint has been applied at a high film build. Painting at high temperatures or above the recommended film build can create air voids. These become points of weakness from which cohesive failures is likely to begin and provides a space for moisture to penetrate into. Both film build and temperature were seen to contribute to the formation of air voids when tested.
In general, splitting is an indication that the integrity of the paint film is failing therefore simply repairing the areas where splitting can be seen is unlikely to cure the problem. Once splitting occurs, it is likely that more will appear. In order to fix the problem completely the old paint will need to be stripped back to sound substrate before any new painting commences and instructions on preparation and application of a suitable primer strictly followed.
Avoid over application – It is quite easy when applying paint with a sprayer to apply more than the recommended film build. However, when paint is applied too thickly, it may not perform as expected. It is important to follow the instructions on product packaging regarding spray rate. Care is required not to over apply the product; brush and roller application is often recommended as these will provide better control of film builds.
Avoid applying paint to hot surfaces – Roof painting is done when the weather is fine, often during summertime, and combined with the inherent high exposure of roofs the surface temperature can get very hot. Although this will lead to fast drying, the integrity of the film could be compromised. When painting a roof, it is recommended to avoid painting during the hottest parts of the day and cooling the surface if necessary with water beforehand. If the roof feels warm, then it is probably too hot to paint it.
