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The Symptoms

Stains can occur for many reasons on any swimming pool surface. In this specific situation, stains usually appear about two to three years after installation of the vinyl liner. A brown stain appears in one or more corners of the pool which are exposed to sunlight. The stain is always at or above the waterline. In an attempt to remove the stain, various cleaning agents (and sometimes abrasives) are tried by the consumer, but little or no results are usually observed. The stain can remain in one place or may extend around the rim of the pool.


The Causes

The causes of this brown discoloration are multifaceted and interrelated. Three factors are known to contribute to the problem: sunlight (specifically ultraviolet radiation and heat), stretching of the liner, and cleaning techniques.


Sunlight contains a wide spectrum of energy including ultraviolet radiation. This energy has the ability to cause various chemical reactions to occur. Fortunately, vinyl formulations include various radiation inhibitors to help slow the reactions which can lead to premature degradation of the vinyl liner. A number of years ago it was recognised by various manufacturers that liners which had been installed five to ten percent undersized to create "a tight fitting" liner experienced more discoloration than liners which had been installed with zero to two percent undersizing. The undersizing created more stretching, particularly in the corners of the pool, and acted to "open up" the structure of the vinyl itself. This stretching creates a thinner layer of sun protection, making the liner more susceptible to both UV and chemical attack. This attack manifests itself as the brown discoloration above the waterline.


In addition, as pool owners saw these stains forming they tried to remove them. Cleaning agents will not remove these stains as it is part of the structure of the vinyl itself. Often unsuccessful attempts to clean the liner led consumers to use abrasive pads and harsher cleaning agents. These actions can remove the protective coating totally and allow further degradation. The effect which occurs is not unlike a zipper being unzipped. Damage occurs in an area, chemical bonds are broken, actions are taken which actually increase the damage, and then discoloration extends all along the waterline as the chemical structure of the vinyl is changed and "unzipped." Unfortunately, the best that a pool owner can hope for in this situation is to prevent the spread of the stain BY DOING NOTHING.


Bio-Lab recommends the use of BioGuard Off The Wall and non-abrasive applicators (soft vinyl brushes, sponges, etc.) for the removal of any stain on a vinyl liner. If this is not effective, the stain may be permanent and no further action should be taken.




  • A brown stain starting in the corner exposed to sunlight above the waterline.
  • Attempts at removal are ineffective, even when the consumer has used abrasive agents.
  • The stain may spread all along the rim of the pool, even if not exposed to direct sunlight.



  • Sunlight, in combination with a stretched liner, causes various chemical reactions to occur which create a brown stain.
  • Harsh cleaning procedures involving abrasives and cleaning agents increase the likelihood of the stain spreading around the pool.



  • In attempts to clean any stain from a vinyl liner, use only recommended cleaners and NEVER use abrasives of any kind.
  • The stain may NOT, unfortunately, be removable. Therefore, make no further attempts as the situation may worsen.


All information supplied by Bio-Lab represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers and agents. It is not to be taken out of context nor used as support for any other claim not made herein.