An outdoor coating on a residential property exhibits discolouration four years after application. One Eighty was assigned to investigate.
The client reported the following:
- The stone finish coating was first applied in 2011 using Binder A.
- The coating failed by swelling and blistering.
- The coating was found not to be inert to water and was scraped down.
- Primer B was applied and discolouration was then noticed after four years.
The discolouration results in a change in the surface roughness of the wall.
The first step in determining the cause for the discolouration was to establish if the primer cured correctly. In order to do so, a differential scanning calorimetry (DSC) was conducted on a discoloured sample (failed sample) and a non-discoloured sample (intact sample).
Differential scanning calorimetry (DSC)
Differential scanning calorimetry allows graphical observation of the latent heat of energy during crystallisation of the primer. Thus, we can conclusively determine if the primer cured correctly.
Scanning electron microscopy (SEM)
SEM allows observation of samples at high resolution to observe characteristic features of under-curing or fracture.
The failed sample has a smaller latent heat of energy (area under endothermic peak) than the intact sample (Figure 2). This suggests that the failed sample includes the under-cured primer. Under-curing results in a loss of adhesion of the primer itself, as well as additional coatings applied on the primer. Lack of adhesion results in the swelling of the primer and coating system, causing a change in surface roughness and uniformity.
The surfaces of the samples differ considerably, where the failed sample appears significantly more porous. This difference in porosity confirms that correct adhesion and spreading of the coating was not achieved during application. Incorrect curing of the primer did not allow the coating to adhere correctly, resulting in an uneven distribution. The discoloured coating sample appears flakier, which results in a higher surface roughness.
- Primer B did not cure completely because of its application on an unclean surface. It is highly likely that residue from Primer A was present. The latter is hydrophilic and therefore repels Primer A, causing an uneven distribution on the wall surface.
- Since Primer A did not adhere correctly, the stone finish coating did not adhere correctly on the entire surface of the outer walls. The loss of adhesion results in the coating swelling over time. This swelling causes chemical changes which result in a colour change.