When stains appear on pool liners or plaster surfaces, they typically fall into three categories: metals, organic matter, or mineral buildup. For example, iron often leaves brown or rust-colored marks, copper can create blue-green discoloration, and calcium deposits show up as white or gray scaling. Determining the source is critical before beginning treatment. Treating a metal stain like organic staining (or vice versa) can waste chemicals, damage surfaces, and prolong downtime.

Simple field checks—such as testing a spot to see how it reacts—can help narrow down the cause. Newer stains are often manageable with the correct chemical approach, while long-standing discoloration may signal surface damage that requires more extensive repair. In most cases, consistent, balanced water chemistry is the best defense against staining in the first place.

Stains on a pool liner or plaster surfaces go beyond the cosmetic. In commercial, public, and community pools, visible staining can trigger swimmer complaints, raise questions during inspections, and shorten surface lifespan if not addressed properly.

Ahead, how to understand what type of stain you’re dealing with plus the steps you need to take to restore appearance and prevent recurrence.

Common Causes of Pool Surface Stains

Before treating a stain, it helps to understand its origin. As we mentioned, in most pools, discoloration is caused by metals, organic matter, or mineral deposits.

1. Metal Stains in Pool Water

Metal stains are extremely common in commercial facilities.

Metals enter pools through:

• Fill water (especially well systems)
• Corroded heaters or plumbing
• Copper-based algaecides
• Corrosive pool water dissolving metal components

When chlorine oxidizes dissolved metals like iron or copper, they can attach to vinyl liners or plaster surfaces.

Common colors:

• Green or turquoise (copper)
• Brown or rust-colored (iron)
• Purple or dark gray (manganese)

Metal stains often appear suddenly after shocking.

2. Organic Stains on Pool Surfaces

Organic stains on pool liners or plaster come from leaves, debris, algae, or other plant material sitting on the pool surface.

What they look like:

• Brown, green, or tannin-colored spots
• Often located near steps, corners, or areas where debris collects

Unlike metal stains, organic stains often lighten when exposed to chlorine.

3. Calcium Scale and Mineral Deposits

In plaster pools especially, high calcium hardness and high pH can lead to scale formation, as well as shorten equipment life and stain surfaces.

What it looks like:

• White, gray, or rough deposits
• Crusty texture
• Often near waterline tile

Scale isn’t technically a stain (it’s mineral buildup) but it can trap metals and dirt, creating discoloration.

How to Identify the Type of Pool Stain

Correct identification prevents unnecessary chemical treatments. In addition to making visual observations, operators commonly use simple field diagnostics, such as a vitamin C tablet test for iron or a chlorine spot test for organic staining, before proceeding with full treatment.

It’s important to always spot test small areas before treating the entire pool.

The Vitamin C Test (For Metal Stains)

Place a vitamin C tablet (absorbic acid) against the stained area for 30–60 seconds.

• If the stain lightens or disappears, it’s likely iron or another metal.
• No change? It’s probably not metal-related.

The Chlorine Test (For Organic Stains)

Apply a small amount of chlorine directly to the stained area.

• If the stain fades or dissolves, it’s likely organic.
• No change suggests metal or scale.

Always test in a small, controlled area first, especially on vinyl liners.

Once you’ve performed these tests, you want to validate your observations against pool water chemistry records. Review your logbooks for recent water test results and any chemical adjustments. Track the pH: a low pH will cause metal corrosion, while a high pH will cause scaling. Also review records for any recent changes, such as chemical treatments or new metal equipment added before the staining.

You can also feel the stains’ texture for additional data (always while wearing gloves). Scale will have a rough or crusty texture, while metal or organic stains are smooth.

Removing Stains from Pool Liner vs. Plaster

When you’ve identified the cause of the stain and are ready to treat, the type of surface will also dictate how you’re treating stains.

Removing Stains from a Vinyl Pool Liner

Vinyl is more delicate. Avoid aggressive brushing, pumice stones, or abrasive tools.

For vinyl:

• Use stain removers specifically labeled safe for liners.
• Lower chlorine before applying certain metal treatments (especially ascorbic acid).
• Spot-treat stains rather than broadcasting product when possible.
• Use a soft brush or sponge to gently agitate treated areas.
• Avoid draining unless manufacturer guidelines allow it.

Improper treatment can cause fading, wrinkling, or permanent liner damage. Always rebalance water after treatment to prevent stains from returning. Document pre- and post-treatment chemical readings to ensure water balance remains within required operating parameters.

Removing Stains from Pool Plaster

Plaster is more durable but also more reactive to water chemistry.

For plaster:

• Metal stains often respond to ascorbic acid treatment followed by a quality sequestrant.
• Organic stains may lift with targeted chlorine treatment and brushing.
• Calcium scale can sometimes be reduced with scale inhibitors or, in severe cases, professional acid washing.
• Check and correct water’s Langelier Saturation Index (LSI) before and after treatment to prevent recurrence.

Persistent staining often signals ongoing chemistry imbalance. Fresher stains respond better to treatment. Unfortunately, etched or deeply embedded stains may require resurfacing. Early identification and correction is always less disruptive—and less expensive—than waiting. Treatment steps and water chemistry adjustments should be logged in daily operator records to support compliance and surface longevity.

Preventing Stains on Pool Surfaces

Stain removal can require partial closure or chemical adjustments that impact operations. Prevention is an operator’s best tool.

Maintain Balanced Water Chemistry

• Keep pH within recommended range
• Maintain proper alkalinity
• Monitor calcium hardness
• Use LSI to confirm water is balanced

Manage Metals Proactively

• Test fill water for iron and copper.
• Use metal sequestrants when needed.
• Inspect heaters and plumbing annually.

Control Debris and Circulation

• Brush surfaces at least weekly
• Vacuum debris promptly
• Ensure circulation reaches steps and corners

Final Takeaway for Operators

Stains on pool liner or plaster are usually symptoms of a larger issue, whether it’s metals, organic buildup, or unbalanced water chemistry. For commercial and public pool operators, identifying the root cause quickly prevents unnecessary chemical use, equipment damage, and facility downtime.

Consistent documentation of water also helps identify trends before staining develops.

Pool Shark H2O replaces traditional paper commercial pool logs with a simple, easy to implement and manage cloud-based solution that brings you and your team confidence that your pools are being managed right. Simply enter pool chemical test results into the Pool Shark H2O app for instant water analysis and chemical dosage recommendations. Your results are logged automatically, while notifications remind staff when pools need to be tested and alerts inform managers and staff when pools haven’t been tested or need attention.

Balanced water protects surfaces. Balanced operations protect reputation.