Flaking paint is most often due to the paint system having inadequate adhesion and flexibility. Both are common problems with lower quality paints. High quality paint systems are engineered for certain conditions and will include the necessary flexibility and adhesion properties to avoid cracking and flaking. Cracking is a problem with 'cohesion' that leads to flaking, which is a problem with 'adhesion'.
Moisture Entering through Cracks in the Paint Film
Flaking is a secondary problem caused by cracking. Cracks in a paint film allow moisture to enter the substrate and break down the adhesion bond. The coating loses it's grip on the substrate and begins to flake.
The Paint System is Old and Needs Replacing
No paint system can last forever. Cracking and flaking will inevitably occur with any paint system after decades of weather and wear and tear. Premature flaking can be avoided by following the correct preparation and applications instructions. The cohesive bond strength (clinging together of like substances) within a coating continues to develop over the lifetime of a coating and at some point it can overwhelm the adhesive strength (clinging together of unlike substances) of the coating. This is usually indicated when the flaking paint is curling.
An Incorrect Paint System was Used
An unsuitable topcoat may have been used that was not engineered to withstand the conditions. The customer may have used an interior paint on an exterior project for example. Oil-based coatings have poorer resistance to UV light and are generally not suitable for exterior applications. Entry-level water-based paints usually contain a cheaper vinyl resin which has poor UV resistance when compared to the 100% acrylic resin found in high-quality water-based paints.
Poor Surface Preparation
If a substrate has not been prepared or cleaned properly, the paint system does not achieve adequate adhesion to the substrate. The poorly prepared substrate has not enabled good adhesion of the prepcoat and has resulted in failure of the paint system. It is also crucial that the substrate is free from moisture when the paint system is applied. Moisture trapped in the substrate will result in multiple problems for the paint system during the drying process and the life of the paint.
No Prepcoat was Used
The topcoat was applied directly to the substrate without a suitable prepcoat. The topcoat is not designed to achieve good adhesion with the substrate and has resulted in failure of the paint system.
An Incorrect Prepcoat was Used
The prepcoat used to prime and seal the surface was not suitable for the project. You may have used a plasterboard prepcoat on a timber substrate for example. The prepcoat was not able to seal the substrate or achieve good adhesion and has resulted in failure of the paint system.
The Prepcoat was Exposed to the Weather
When a prepcoat is applied to a substrate, it is important that it be coated soon after with a suitable topcoat. The prepcoat should not be exposed to weather or excessive UV light as it is not designed to withstand these conditions, and the long-term performance of the prepcoat will be compromised.
The Prepcoat or Topcoat was applied Incorrectly
Incorrect application of the prepcoat or topcoat will compromise the long-term performance of the paint system. Cracking and flaking can result when paint is applied too thinly due to overspreading (higher-than-recommended spreading rate) or excessive thinning. These practices tend to diminish the paint's final film thickness, so that it is more vulnerable to cracking and flaking. The ambient weather conditions may have been unsuitable when the paint was applied. The coating may been applied in too high or low temperatures, or extreme humidities. The coats may also have not been given sufficient drying times.
The Substrate has Moved, Expanded, Contracted or Cracked
When timber and other substrates are involved, moisture intrusion results in swelling of the wood surface followed by contraction as the wood dries. The expansion and contraction cycles, often aggravated by temperature cycles, can result in cracking and subsequent flaking.
High Humidity / Ambient Moisture while Painting
Drying paint that is exposed to high humidity, dew or rain will not cure correctly and will have a shortened lifespan. Cracking and flaking are more likely to occur earlier. This is because the water solvent is unable to evaporate at its usual rate, as the ambient air is already filled with moisture. This alters the drying process and prevents the coalescing solvent from perform its curing function. If a storm is expected, avoid painting for a total of eight hours: the four hours preceding its arrival, and the four hours that follow. As explained in the Water-Based Coatings - Advanced section, this specific problem generally only applies to water-based coatings, as the oil-based drying process is not as sensitive to ambient moisture.
High Humidity / Ambient Moisture Shortly after Drying
Exposing a water-based paint film to dew, high-humidity or rain shortly after the paint has dried can also cause permanent damage to the paint system. As the coating has dried but not yet fully cured, ambient moisture can return into the coating, preventing the important coalescing process from completing as intended. As explained in the Water-Based Coatings - Advanced section, this specific problem generally only applies to water-based coatings, as the oil-based drying process is not as sensitive to ambient moisture. Once a water-based coating has fully cured, the coating is then ‘water-resistant’ and this is no longer an issue.