Why is my plaster turning black?

Author: Mr. Cooper Hyatt  |  Last update: Saturday, September 13, 2025

Late hard troweling can cause “trowel burn” which darkens the plaster color in localized areas. Sanding can remove this discoloration.

How do I get my pool plaster white again?

Pouring a bit of pool shock directly onto a stain works similar to bleach and is ideal for plaster surfaces, but a bit harsh for vinyl liners. The Stain Eraser is another great stain removal product that does not require the use of chemicals and is great for smaller stains and stains in tough corners.

What causes staining on plaster?

Too fast curing, typical for the summer conditions, contributes to discolouration, caused by incorrect bonding of the plaster. Contrary to popular belief, the purpose is not to dry the plaster but its chemical curing, which requires relevant temperature, moisture and time.

Why are the walls of my pool turning black?

That is black algae. It is the worst type of algae you can get in your pool. The best way to eradicate it is; get a pair of rubber gloves, put them on, grab a 3 inch chlorine tablet and rub it right on top of that black spot. You'll find that it's real slimy and you have to scrub that slimy head off.

What causes pool plaster discoloration?

Discolorations: Discolorations can occur in a new pool plaster due to the addition of excessive calcium chloride set accelerator. Mottled discoloration can be severe that is difficult to remove or discolorations can be in the form of “trowel burn” sanding, acid washing, or torching the surface.

The Ugly Truth About Black Mould

Why does plaster turn black?

Dyes always cure at various rates in plaster and can darken due to that. The most common cause is calcium chloride. If too much was used in the mix or it wasn't mixed in properly it will also cause blotchiness. The third and other common reason is improper troweling during the finish process.

Can too much chlorine damage plaster?

The third step to care for your pool plaster is to control the chlorine levels in your pool. Chlorine can cause discoloration and etching of your plaster if it is not properly balanced.

What is the black mold on my plaster pool?

Black Spot is typically caused by poor chlorination or high phosphate levels. The algae can feed on the phosphates, stimulating its growth. However, Black Spot has been known to survive even when there are low phosphate levels in the pool water. The key is keeping the water balanced with adequate chlorination.

Why did my pool turn black overnight?

Like all algae, black algae flourish in water, where they can photosynthesize and easily multiply. Growth is particularly common in pools with pH imbalances, low chlorine levels, and poor water circulation and filtration.

How do I remove black stains from my pool?

A nylon brush can break down the protective layers of the black algae, and chlorine tablets and pool shock chemicals can ensure that your pool water is balanced (to prevent black algae from coming back, you'll want a pH between 7.4 and 7.6).

What is the black stuff on plaster?

Black Mould on walls (Stachybotrys chartarum, Aspergillus niger or others) is often a sign of a serious problem in your home that needs to be thoroughly investigated. It is often caused by excess condensation, usually as a result of a lack of ventilation in the home that results in high humidity levels.

How can you tell if plaster is bad?

Cracking, bulging, or uneven surfaces, poor adhesion, mould and moisture, an unappealing appearance, insufficient strength, and a poor finish are just some of the problems that can arise from sloppy wall plastering.

What are the three main plaster defects?

Common Plaster Defects and their Cure
  • Blistering of Plastered Surface. ...
  • Plaster De-bonding. ...
  • Cracks on Plastered Surface. ...
  • Efflorescence on Plastered Surface. ...
  • Falling Out of Plaster. ...
  • Popping of Plaster. ...
  • Loose Plaster.

What is the life of pool plaster?

Standard white plaster will do what it is supposed to. It will be watertight and serve its function for that 10 to 15 years depending on maintenance. But pebble finishes, properly installed and maintained, will last 20 to 30 years or more.

Can muriatic acid damage pool plaster?

It can shorten the lifespan of plaster finishes. Muriatic Acid, the chemical commonly used is dangerous and corrosive, so the application should be done by a professional. However, Muriatic Acid does not damage quartz or pebble aggregate finishes.

What color should pool plaster be?

What is the most popular pool plaster color? White is the most popular pool plaster color because it makes the pool water look bright, clear and is the easiest to clean, and resists fading compared to other colors, especially darker.

What turns a pool black?

What is the cause of black swimming pool water? Black-colored pool water is not a frequent pool problem in a well-maintained pool. Black water can be caused by either a very high level of organic debris, such as that found in abandoned pools, or metals, such as manganese.

Why are my pool walls black?

The algal spores often take up residence on your pool walls and floor when it is empty and grow once it has been refilled with water. Algae growth is caused by an imbalance in the pH of the pool from wind and rain, so maintaining regular chemical treatments will help keep black spot at bay.

Is black pool normal?

Healthcare providers diagnose melena when they've confirmed that the color of your poop is, in fact, from internal bleeding. Black stool is often a sign of gastrointestinal bleeding, but not always. Certain medications, supplements and foods may also turn your poop black.

What kills black mold on plaster?

Plasterboard walls

As plasterboard is constructed of layers, using bleach or even a mould wash will only remove the surface mould. The main issue is active mould growing throughout the plasterboard. Unfortunately, the only way to get rid of mould for good is to replace the plasterboard.

How do you remove black mold from plaster?

White vinegar

Mix up a solution of 1 part vinegar to 1 part water in a spray bottle. Apply this spray to the mouldy area and leave it to work for an hour. You can then wipe the vinegar away using a damp cloth which should hopefully remove the mould.

How do you treat a black pool?

7 Tips To Get Rid Of Black Algae
  1. Test Your Pool Levels Twice Weekly. ...
  2. Sanitize All Pool Related Items. ...
  3. Begin to Scrub the Black Algae. ...
  4. Add an Algaecide to the Pool. ...
  5. Triple Shock Your Pool & Scrub Again. ...
  6. Vaccum. ...
  7. Keep Your Water Moving.

What are the symptoms of too much chlorine in a pool?

The effect of excess chlorine in pools is an alteration in pH levels of pool water, making it more acidic. Signs of high chlorine levels can include itchy sensations in the eyes, nose and throat, nausea, vomiting, dizziness, lung irritation, and skin rashes.

How do you clean a plaster pool?

After draining your swimming pool, change into protective clothing including boots, gloves, and goggles. Then, combine some muriatic acid and water into an acid washing device. Next, spray the pool wall with hose water until it is saturated and then apply the acid mixture to the wall for about 30 seconds.

Does bleach damage plaster?

A: No, avoid using bleach or harsh chemicals, as they can damage the plaster finish. Q: Can I use a pressure washer on Venetian plaster? A: It's not recommended, as the high pressure can cause irreparable damage.

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