Pool Plaster Stains and Calcium Scaling
A free referral service connecting Pinellas homeowners with licensed pool contractors.
The color of a stain is a clue. Here's what the National Plasterers Council says each one usually points to, and how to keep scale away.
What the color of a stain can mean
| What you see | Common cause | Source |
|---|---|---|
| White or light film that dulls the color | Surface scaling | National Plasterers Council |
| Blue, green or black stains | Copper or manganese | National Plasterers Council |
| Red, orange or black stains | Iron | National Plasterers Council |
| Small rust spots | Fertilizer, which contains iron and other minerals | Pebble Tec |
What causes calcium scaling
Scale is minerals or metals that come out of the water and stick to tile and finishes, causing stains and roughness. The National Plasterers Council ties it to high pH or high alkalinity, calcium hardness or dissolved solids.
Pebble Tec calls scaling, a white haze, the most common maintenance issue on its finishes, and says it comes from poor water chemistry.
St. Petersburg's tap water averaged pH 8.2 in 2025. Test your pool water after filling. Pinellas water by utility.
How to help prevent stains and scale
- Keep pH between 7.2 and 7.8. Above 7.8, water tends to scale or stain.
- Keep calcium hardness between 200 and 400 ppm.
- Test often. High pH can cause scale and discoloration; low pH can etch.
- Keep lawn fertilizer out of the pool.
About stain-control chemicals
Chelates and sequestrants can counteract metals and minerals. The National Plasterers Council warns that once you start them, you have to keep using them.
If scale or stains won't come out, it may be time for a new finish. Why pool surfaces get rough ยท Pool resurfacing
Questions about pool stains and scale
Can calcium scale be removed from a pool finish?
Pebble Tec says the most common method is chemical treatment. Sometimes the pool has to be drained to remove scale with chemicals or a pressure washer. It also notes scale removal isn't covered by its warranty.
What causes green or black stains in a pool?
Metals are a common cause. The National Plasterers Council says copper and manganese can cause blue, green or black stains, and iron can cause red, orange or black stains.
Stained or scaly? Get a quote
Tell us about your pool. Your request comes to us. We refer it to a Florida-licensed pool contractor.
Sources
- City of St. Petersburg, 2025 Water Quality Report
- National Plasterers Council, Glossary
- National Plasterers Council, Water Chemistry
- National Plasterers Council, What Happened to My Pool's Interior Finish Color? (white paper)
- National Plasterers Council, The Importance of Water Testing (white paper)
- Pebble Tec, Support and FAQs