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Project Guide - Cracking

What is Cracking?

Cracking is the splitting of a dried, brittle paint film. Cracking can affect one or more coats. Apart from being unsightly, cracking is a failure of one or more coats of the applied paint system. Cracking is an earlier stage of the flaking problem.

What Causes Cracking?

Cracking 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 eventual flaking.

The Paint System is Old and Needs Replacing
No paint system can last forever. Failure will inevitably occur with any paint system after decades of weather, wear and tear. Premature cracking can be avoided by using quality paint products, and following the correct preparation and applications instructions.

Poor Surface Preparation
If a substrate has not been prepared or cleaned properly, the paint system does not achieve adequate adhesion to the substrate. It is also crucial that the substrate is free from moisture when the paint system is applied. Moisture on the surface of a substrate during application will affect the adhesion process. Moisture trapped within 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 was not able to achieve a 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. This can occur if you have used a plasterboard prepcoat on a timber substrate for example. The prepcoat was not able to achieve a good adhesion to the substrate and this 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.

An Incorrect Topcoat was Used
An inappropriate topcoat may have been used that was not engineered to withstand the conditions. You may have used an interior paint on an exterior project for example.

The Prepcoat or Topcoat were applied Incorrectly
Incorrect application of the prepcoat or topcoat will compromise the long-term performance of the paint system. Cracking and subsequent flaking can result when paint is applied too thinly due to overspreading (higher-than-recommended spreading rate) or excessive thinning. These practices diminish the paint's final film thickness, so that it is more vulnerable to cracking. 

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, further aggravated by freeze-thaw 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 is more likely to occur earlier as a result. 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, refrain from 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 is more likely to apply 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 and cracking is more likely to occur as a result. As the coating has dried but not yet fully cured, ambient moisture can return into the coating, preventing the important coalescing process from occurring 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 ‘weather-resistant’ and this is no longer an issue. 

Note: Oil-based paints are more brittle than water-based paints and are more susceptible to cracking. Oil-based paints are not recommended for exterior use.

How to Prevent Cracking?

In order to prevent cracking:

  • Ensure the substrate is cleaned and prepared according to the manufacturers instructions.
  • Ensure the paint is applied according to the manufacturer's spreading rate, and the sufficient drying time is allowed for each prepcoat and topcoat layer. 
  • Ensure the correct prepcoat and topcoat system is used for the conditions.
  • Ensure the prepcoat is not left exposed to ambient conditions before topcoat is applied.
  • Do not paint in high temperatures, or very low humidity. 
  • Do not paint in high humidity or during rain or when rain is forecast. 
  • Make sure the freshly applied coating is not exposed to dew, rain or high-humidity shortly after it has dried.

How to Solve the Problem?

Cracks allow moisture to enter the lower coatings or the substrate, eventually resulting the secondary problem of flaking. A coating that has cracked or flaked will need to be removed by scraping or sanding. If only a small section of the surface has flaked then it may be possible to repair only that section. A cross-hatch adhesion test should be carried out across the rest of the surface to determine if it is in sound condition.

The substrate will need to be prepared and re-painted with a quality prepcoat and a topcoat system. The paint must be applied according to the manufacturers spreading rate and sufficient drying time is required for all coats. You must wait until suitable ambient conditions are available for painting.