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How Can Stains Be Removed From a Gunite Pool?

Backyard swimming pool
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Gunite is a type of concrete that is especially prepared and shot through a concrete blaster system with a high-powered nozzle. Gunite requires skill to apply correctly, but coats curving surfaces where traditional concrete laying practices don't work. Gunite is a process where dry concrete and water that are mixed together at the last moment. Gunite pools are overlaid with waterproof plaster.

Stain Causes and Prevention

Stains in the plaster of concrete-built swimming pools grow due to minerals in the water, which react due to the chemical reactions of the concrete meeting the pool water for the first time. While the concrete may be cured in terms of being complete dry, it still needs to be cured with its exposure to the pool water. Until this happens--it may take up to several months--stains will form more easily on the plaster, because particles in the water will be more attracted to the surface. This is caused mostly by the differences in the pH levels of the two substances, which will fade away with time as atoms are exchanged between the two and the surface layer of the plaster changes to meet the average pH level of the water in the pool

Sometimes you may confuse mineral stains and algae stains. The algae found in pools will typically be green in shade, sometimes red. Any other color, especially a color that changes in hue and becomes darker, is probably a mineral-based deposit. You can clean off these deposits, but they can also be prevented by using the softest water available to you, the water most devoid of supersaturated minerals like limestone or calcium. Use mineral treatments to change the alkalinity of the water and make it harder for the minerals to form deposits on the plaster, or install a metal particle filter in your pool system. If possible, avoid using copper pipes or any sort of metal that can corrode over time.

Cleaning Stains

If you have white crystals forming on the plaster, you probably you have calcium deposits. If you have gray or dark colored stains, then fiberglass or iron particles may be the culprit. Copper may be causing bluish-green stains, while manganese typically creates pink or red patches. If any of these stains start appearing, it is important that you adjust the alkaline levels of your pool to around 7.4 or 7.8 on the pH scale. Adust the calcium hardness of the water to around 200 to 400 ppm, to avoid calcium build-up. You should remove any source of the mineral deposits, such as copper pipes or possible groundwater contamination, should be removed. If you are unsure whether the stain is algae or mineral-based and it is difficult to remove or recurring, contact an expect to diagnose the problem.

Once mineral stains are present on the plaster, they are difficult to remove with simple mineral treatments. Most pools are professionally whitened using acid cleaners from time to time. You can try to find an ascorbic acid cleaner at a local pool store and use it to clean the stains, but it cannot be mixed with chlorine, so dechlorinated the pool, or empty and clean it. Always keep in mind there may be another cause to the stains, such as the plaster wearing off and exposing the gunite beneath.

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