Shedding Light on the Enemy of Fibreglass: Osmosis

Owning a pool can be a significant commitment, requiring annual maintenance and regular upkeep to maintain an ideal atmosphere for relaxation. Most pool owners know about some of the more common problems, like chemical imbalances and filtration system malfunctions, but are not aware of the potential issues that can come about as a pool ages. For pools that are ten to fifteen years old, issues may arise in the fiberglass surface, leaving pool owners flummoxed by the raised bumps and dark-colored blisters that develop. The cause? Osmosis.

Fibreglass Osmosis Bubbles

Here we have a pool that has osmosis behind the watertight seal.

Osmosis and Swimming Pools

The scientific definition of osmosis, which refers to the movement of molecules through a semi-permanent membrane, is a basic concept in high school biology, but the application of the term as it applies to pool surfaces is far less commonly known. In essence, osmosis, also known as black plague, refers to bumps or blisters that appear on a pool’s surface based on a failed watertight seal.

These imperfections exist permanently, or may come and go over time. The bumps that arise are often one to four inches in diameter, feel hard or porous to the touch, and may not pop or crack when stepped on or pressed. Some may leak a brown or black tar-like substance, causing alarm in pool owners concerned with possible safety consequences. Treatments like staining or popping will not diminish the effects, and shocking with high doses of chlorine can only exacerbate the problem.

Osmosis Signs and Causes

Grinding epoxy filler to eliminate osmosis

One of our teammates, using a grinder to smooth the epoxy filler we use to eliminate decay left by osmosis.

Osmosis occurs when air and water seep through a pool’s lining, an issue often caused by poor consolidation of laminate or the presence of an unreacted cobalt accelerator. These issues can arise in any number of ways, but are generally related to the installation and care of a pool.

Faulty Construction Techniques

Many osmosis issues have to do with the quality or kind of laminate or poolcoat placed on the pool’s interior surface, and are often connected to the outdated technology used to seal pools over the last several decades. Most fiberglass pools are coated with a cloth mat soaked in resin that is covered by a gel coat. This construction technique isn’t perfect, however, leading to water saturation in portions of the cloth mat. When this occurs, the brownish black blisters arise, separating the gel coat from the mat and creating an expensive fix for pool owners.

Over the last several years, however, manufacturers began including an additional layer of vinylester resin between the mat and gel coat in order to provide a watertight barrier. This addition provides an extra layer of protection, preventing water from seeping through to the cloth mat. This is most effective when gelcoat is sufficiently thick enough, and the gelcoat is allowed to cure for the proper amount of time; otherwise, even an enforced lining can succumb to osmosis damage.

Poor Maintenance

Another successful pool resurfacing

Another New South Wales pool saved from osmosis by the outstanding Fibrestyle team!

When maintenance and operating conditions of a pool are ignored, osmosis is far more likely. Excessive exposure to heat can speed up the process; as such, pools kept at a high temperature are much more likely to suffer consequences. Pools expected to be kept warmer than 86 degrees Fahrenheit at a regular basis should be constructed with heat and chemical resistant resins rather than standard products.

Chemical use can also be a factor. When chlorine levels are too high, especially in conjunction with high temperatures, black plague is more likely to occur. Pool shock can be an especially dangerous factor; increased use of shock can rapidly accelerate the process, leading to osmosis signs years ahead of schedule. To reduce this risk, ensure your pool’s pumps are functioning normally, and track both chlorine and pH regularly.

Osmosis is an unwelcome sign in any pool, but understanding what you’re up against can help you find the right fix. At first sign of black plague in your pool, it’s important to seek professional attention to diagnose your issues and provide an appropriate remedy, bringing your pool back up to par as quickly and affordably as possible. If you think you may have an osmosis problem, give us a call or request a quote and we will investigate any issues you may be having.

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Derek is the owner and manager of Fibrestyle Pool Resurfacing. Located in New South Wales, Fibrestyle has been proudly serving the Sydney area with an unwavering focus on quality for over 10 years. Derek is passionate about exceeding expectations, and this attitude has been the driver in Fibrestyle's success through the last decade.