Water line or mark stains in a swimming pool, causes, what to do?

Stains around the border of a pool are pretty unsightly and can ruin an otherwise functional pool. Stains can be caused by poor maintenance, or things beyond the homeowners control, such as heavy metals in the water.

It is very important to ensure that the pH and Chlorine levels are kept to their correct parameters, no matter what type of pool.

There are 4 main types of swimming pools in Spain, here we look at what causes waterline staining in each type of swimming pool and look at what can be done to solve it.

Tiled swimming pools

Glass mosaic tiles used in swimming pools are very hard wearing, but eventually they succumb to staining. The staining is mostly caused by a build up of cal. The cal etches into the surface of the glass mosaic tile, making it rougher, once it starts to loose its shiny finish, the process repeats, but with the cal being harder to remove.

Once the cal really starts to stick, it can then become stained brown from sun cream, fatty deposits from skin or brown from minerals in the water. Yellow staining can again be caused by sun cream and pollen. Blue or purple staining is normally due to too much copper in the water, which binds with the cal.

The solution here is to ensure that people shower before swimming and to regularly clean the scum line with a brush and cleaning fluid. If the cal becomes thicker, it can be removed using hydrochloric acid. If this fails, then the tiles can be broken off and replaced, but this is costly and requires that the pool is drained.

Painted pools

Here the problems are broadly similar that above, but the calcium is not so much of a problem. The scum line can be cleaned off using detergent, such as washing up liquid. This will removed the greasy deposits which give the pool border a horrid look.

If this all fails, the solution is to either repaint the border area, or the whole pool.

Prefabricated Fibreglass / Polyester pools or Pools that have been fibre glassed after.

Fibre glass pools have their waterline damaged again by calcium build up etching into the surface, then giving the staining somewhere to take hold. This is normally more of a problem in pools that have been treated with fibreglass after they were built, as this finish does not set as hard as that in pools prefabricated in a factory.

Some of this staining can be removed with detergent and a soft sponge, if this does not work, then a weak solution of “agua fuerte” can be used.

If the staining can not be removed this way, then the area must be re treated with fibreglass, polyester resin and gelcoat, which means emptying the pool.

Liner Pools

In liner pools, there are 2 types of lining. There are thin bag liners, which have no reinforcing or treatment against UV or chemical damage.

These bag liners can get purple stains in the waterline, from copper in the water, brown stains from suncream and yellow from pollen.

It is very difficult to clean these stains, as the liner is so thin, but detergent can be used for fatty deposits.

Reinforced liners are treated with lacquer and other layers to protect the colour, so they have a far longer life, they can be cleaned using detergent or specific cleaners, such as Renolit AlkorClean. The most important thing is not to let any stains build up, if the tide line is cleaned using a specific cleaner, then the staining is exponentially reduced.


Renolit Cleaning Solution

If none of this works, what do I do to remove the scum line from the pool border?

Sometimes, no matter how diligent the pool maintenance is, there will be staining, due to age or because the water is of very poor quality and has very high calcium content, or the water has things like high concentrations of iron. After repeated attempts to remove staining, the surface can break down to a point where either the border has to be changed, or you just have to resign yourself to leaving it, or….

You could try one of our self adhesive vinyl swimming pool borders, which will cosmetically lift the border line of the swimming pool, hiding unsightly stains, almost making the pool look like new. These are an easy DIY fit, only requiring that the pool water level is lowered slightly. No special tools are needed to fit the border.

For more information about our swimming pool borders, click.

Pool border cal stain

Cal damage pool border

Tiled pool, Spain, new border

Pool border applied to tiled pool, Spain

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