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Advice for service pros dealing with pesky vinyl liner stains

By Terry Arko

A cover that seals around the pool’s perimeter (pictured) should be used to prevent the accumulation of organic debris such as leaves and insects.

Vinyl liner pools are one of the most popular construction methods in Canada for many reasons—one being ease of maintenance. For example, it is harder for algae to grow on vinyl than on plaster. However, even the highest quality vinyl liner is still subject to staining and discolouration. Improper water balance, water treatments, and specific types of debris can cause persistent maintenance problems, most notably staining on vinyl surfaces. This article provides a brief overview of how typical staining occurs to help service professionals take care of these persistent maintenance challenges.

Black staining

When black staining appears on a vinyl liner, one of the first questions that come to mind is if algae or metals caused it. To determine the cause, there are some simple ways to tell the difference.


Black staining from metals

When black staining appears on a vinyl liner, one of the first questions that come to mind is if algae or metals caused it.

Copper, iron, and manganese can precipitate out of solution resulting in pool liner stains, which generally appear black, brown, or grey. The metals may be introduced into the pool via the fill water or from improper pH causing corrosion of pipes or heaters. The presence of metal staining can be confirmed by treating a small portion of the area with a vitamin C tablet. If the stain has been caused by metal, it will lift from the liner. Any remaining stains can be treated in a similar manner using an ascorbic or citric acid stain lifter. If this does not work, the stain is likely due to an organic source such as black algae. After removing the stains, a metal sequestering product should be used to prevent any further staining.

Black staining from algae

Black algae are tenacious organisms with a chlorine-resistant coating that require a number of steps to remove. They appear as a series of small black spots typically on the liner’s seams, near the steps on the ladder railings, or around light rings. Black algae do not usually grow on the vinyl’s smoother surfaces (i.e. walls and floor).

To remove these stains, first scrub the black algae using a nylon brush. Reducing the pH to a minimum 7.2 will increase the chlorine’s effectiveness. Then, super-chlorinate the pool (normally to 10 parts per million [ppm] free available chlorine) and add a dose of a quaternary (quat) type algaecide. The pH should then be increased to a more neutral range (e.g. 7.4) to prevent the liner from wrinkling.

Mould on pool liners

If stains are still present after treating for metals and black algae, another type of grey/black stain can occur due to the growth of micro-organisms on the backside of the liner. These micro-organisms can produce dyes that are soluble in the plasticizers used to make the vinyl liner pliable. The microbial dye wicks through the liner creating an unsightly, irregular-shaped blotch.

These stains can be diminished on the exterior side of the liner by super chlorinating the water for a period, but they will likely reappear since the stain is occurring on the backside of the liner.

A polyethylene barrier used between the pool walls/floor helps to prevent this type of staining that is caused by these micro-organisms.

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