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A swimming pool installer (with approximately 20 years’ experience installing polyester-glass fibre pools) had noticed that the installed pools start bubbling around two years after installation. These bubbles form below the waterline. Upon further investigation, it was found that the bubbles originate in the barrier coat.

One Eighty was approached to evaluate the barrier coat and determine the root cause of the bubble formation.

One Eighty was supplied with various panels removed from several swimming pools installed since 2014. From these samples, three panels were chosen for the evaluation. Two of the panels were produced with two different catalysts whereas in the 3rd no barrier coat was used (installed in 2017). One Eighty was also supplied with liquid samples of the resins currently in use. These were evaluated to establish their contribution to the problem encountered.


One Eighty thoroughly documented and examined swimming pool construction.

A thorough description of the construction process and the various layers/steps in the process as then drawn up:

  1. Concrete substrate
  2. Adhesion coat
  3. Polyester & Glass Fibre Structure
  4. Barrier Coat
  5. Pool coat

One Eighty also examined in detail the resins used in the manufacture of fibreglass swimming pools and their structures:

  1. Unsaturated Polyester Resins (UPR)
  2. Ortho vs Iso Resins
  3. NPG Resins
  4. Vinyl Ester Resins
  5. VER vs UPR (in a swimming pool environment)

Lastly, One Eighty examined permeability, hydrolysis & Osmotic blisters and the issue of black spots in UPR Pools

Received samples

One Eighty received a total of nine samples (panels were removed from pools that exhibited blisters formation) from seven different jobs of the pool installer.

One Eighty selected three samples to work with based on (a) the catalysts used and (b) the “sealer” coat used. An example of what one of the panels looked like is reflected in the image below:

One Eighty examined how the different panels were prepared for the construction of the swimming pools in question.


Observations and Test Methodology:


Observations from One Eighty:

Each of the panels were visually inspected and the following observations were made:

  1. Panels 1 and 2 were covered with blisters varying in size from 5mm to 25mm whereas the Panel X had no blisters.
  2. Panels 1 and 2 had small black spots scattered over the surface whereas the Panel X did not.
  3. Panels 1 and 2 panels had black spots or streaks at the edges of the blisters and in some cases the black spots were dissected with cracks. The latter varied from 2mm to 40mm.
  4. No black spots, as mentioned in 3, or subsequent cracks were found on Panel 3
  5. All the panels had structurally sound backings and this showed the panels had excellent adhesion to the concrete substrate, i.e. cohesion failure of the substrate and not adhesion failure between the concrete and panel. This would then mean that the blisters are due to osmotic action from the front of the panel (pool side) and not due to hydrostatic osmotic pressure from behind the pool (concrete side).

One of the blisters were cracked open and it expelled a light yellow and slightly viscous liquid. The liquid had a strong acidic smell (a smell similar to acid-curing single pack silicone, i.e. acetic acid).


Observations from the customer:

The customer noted that these blisters formed below the waterline of the pool. This confirms the fifth of One Eighty’s observations.


Test methodology:

Blisters were identified (on each of the 2 panels that had blisters) and sectioned. These blisters were split, and the resulting cross-section inspected with light microscopy as well as scanning electron microscopy (SEM). A small section of the other panel was removed and similarly inspected. Black spots were also identified with the SEM.

One Eighty then carried out the following tests on all three samples – both on the surface and on a cross-section.

Light Microscopy

A cross section of one of the panels



The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample. In most applications, data is collected over a selected area of the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties.

A SEM Micrograph of a cross section of one of the panels


Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). The EDS technique detects x-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the analyzed volume. These x-rays released from the surface of the sample carry a unique energy signature that are specific to elements found in the sample.

SEM/EDS was used to scan and detect elements through a cross-section of the panels around the blisters.


X-ray Fluorescence (XRF) is a technique widely used in elemental analysis. It is based on the principle that an atom’s inner electrons, when bombarded with high energy radiation such as X-rays, are ejected. The atom relaxes by emitting photons of characteristic wavelengths, which are used to identify the element.


From the extensive results of the testing on the surface and cross section of the three samples as described above, One Eighty could conclusively prove:

  • The blister forms between the glass / resin matrix and the barrier coat
  • No pinholes or source of water could be detected on the glass / resin matrix side of the pool installation (i.e. the source is not due to hydrostatic pressure from within the ground)
  • The blister formation is below the water line in the pool
  • he blister is associated with black spots or streaks and a pinhole or a crack
  • The cobalt content in the area of the black spot is up to 30 times more than in the resin /glass matrix
  • Although it is a known fact that the pool coat is a semi-permeable layer and that the VER is used to prevent water reaching the resin / glass matrix, the pool water still reached the glass / resin matrix
  • A white powder was detected on the underside of the blister. This powder could be from filler in the polyester resin but also from the hydrolysis of the glass.

Given the observations made, as well as with the outcome of the tests / analyses performed, it would be safe to say that the barrier either was not functioning properly when installed or degraded in the time after installation.
The pathway for the above breakdown was most likely the pool water’s (and its ingredients) reaction with the cobalt residue in the resin, creating a pathway through the barrier coat, allowing water to contact the glass / resin matrix, hydrolysing this matrix and creating a positive osmotic pressure allowing the some of the broken down material to leach out and more water to move through the coating system into the blister.

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