Gunite and Plaster Pool Service: LSI Targets, Etching vs Scaling, Acid Washing, and CYA Effects on Plaster Chemistry

What LSI Targets Should You Maintain for Plaster Pools?

The Langelier Saturation Index measures whether your pool water is in equilibrium with calcium carbonate. A negative LSI means the water is undersaturated and will dissolve calcium from the plaster to satisfy its demand. A positive LSI means the water is oversaturated and will deposit calcium on surfaces. Both extremes damage the plaster. The target range for plaster pools is -0.3 to +0.3, with 0.0 (perfectly balanced) being ideal.

LSI Input Parameters

The LSI calculation uses five measured parameters: pH, water temperature, calcium hardness, total alkalinity (adjusted for CYA contribution), and TDS. For practical service work, pH and temperature have the largest impact on daily LSI shifts because they change the most. Calcium hardness and alkalinity change slowly over weeks or months. This means your most important daily LSI management tool is pH control.

Ideal Chemistry Ranges for Plaster

How Do You Identify Etching vs Scaling?

Etching and scaling are opposite problems caused by opposite chemistry conditions, but they can look similar to an untrained eye. Correctly identifying which one you are dealing with determines the treatment approach. Treating etching with the same chemistry adjustments as scaling makes the problem worse, and vice versa.

Etching: What It Looks Like

Etching occurs when aggressive, undersaturated water (negative LSI) dissolves calcium from the plaster surface. The plaster becomes rough, pitted, and may develop a mottled or uneven appearance. Run your hand along the pool wall: etched plaster feels like fine sandpaper. In severe cases, the aggregate (pebble or quartz) becomes exposed as the plaster matrix dissolves around it. Etching is irreversible. You cannot add plaster back. You can only stop it from progressing by correcting the water balance.

Scaling: What It Looks Like

Scaling occurs when oversaturated water (positive LSI) deposits calcium carbonate on surfaces. Scaling appears as white, rough deposits on the plaster, tile line, and inside equipment. It can form a uniform haze over the entire surface or concentrate at the waterline and around return fittings where water movement is highest. Scaling makes the surface rough and can trap dirt and algae, making the pool harder to clean. Unlike etching, scaling is partially reversible. Mild scale can be removed through chemistry adjustment and brushing. Heavy scale requires acid treatment.

Diagnostic Comparison

A simple field test: place a few drops of muriatic acid on the affected surface. If it fizzes, you are looking at calcium scale (the acid is dissolving the deposit). If there is no reaction, the surface is etched (the calcium has already been dissolved from the plaster). This takes 10 seconds and gives you a definitive answer.

How and When Should You Acid Wash a Plaster Pool?

Acid washing removes a thin layer of the plaster surface to reveal fresh, clean material underneath. It is an effective treatment for surface staining, mineral deposits, and algae-stained plaster. But it is also a destructive process that permanently removes plaster material, so it should be performed sparingly and with clear customer expectations.

When Acid Washing Is Appropriate

• Heavy staining from metals (iron, copper, manganese) that does not respond to chemical treatment
• Calcium scale buildup that cannot be removed through water chemistry adjustment
• Algae staining embedded in the plaster surface after a severe green pool recovery
• General discoloration and surface grime accumulated over years of service
• Pre-sale cosmetic improvement to maximize property value

When to Avoid Acid Washing

• The plaster is already thin from previous acid washes (check with the customer)
• The plaster is heavily etched (acid washing will make it worse)
• The staining is from organic sources (leaves, tannins) that respond to chlorine treatment
• The pool has been acid washed within the last five years
• The plaster is cracked or delaminating (acid washing will accelerate failure)

Acid Wash Procedure

1. 1Drain the pool completely. Ensure the hydrostatic relief plugs are accessible in case of groundwater pressure.
2. 2Remove all debris from the empty pool. Rinse the surface with a pressure washer to remove loose material.
3. 3Mix muriatic acid with water at a 1:1 ratio in a watering can. Always add acid to water, never water to acid.
4. 4Wet a 10-foot section of the wall with a garden hose.
5. 5Apply the acid solution evenly to the wet surface from top to bottom.
6. 6Allow the acid to work for 30 to 60 seconds. You will see bubbling as it dissolves the surface layer.
7. 7Rinse thoroughly with the hose, directing the acid wash water to the deep end.
8. 8Move to the next section and repeat. Work in sections to prevent the acid from sitting too long in any area.
9. 9After the entire pool is washed, neutralize the acid water in the deep end with soda ash before pumping it out.
10. 10Refill the pool and perform startup chemistry balance per NPC (National Plasterers Council) guidelines.

Acid washing should be performed no more than once every five to seven years. Each wash removes approximately 1/16 inch of plaster material. A standard plaster finish is approximately 3/8 to 1/2 inch thick, which means a pool can handle five to eight acid washes over its lifetime before the plaster becomes too thin and needs resurfacing. Always discuss this finite resource with the customer before proceeding.

How Do You Treat Calcium Stains and Deposits?

Calcium deposits on plaster pools range from light waterline scale to heavy surface encrustation. The treatment approach depends on severity. Light deposits respond to chemistry adjustment and brushing. Moderate deposits require chemical treatment. Heavy deposits may need physical removal or acid washing.

Light Calcium Scale (Waterline and Surface Haze)

Lower the pH to 7.2 and reduce total alkalinity to 70 to 80 ppm. Add a phosphonic acid-based scale inhibitor (chelating agent) per the product dosing instructions. Brush the affected areas vigorously with a stainless steel pool brush. The combination of slightly aggressive water chemistry and physical agitation will dissolve light calcium deposits over one to two weeks. Monitor LSI during this treatment period to ensure you do not push the water into etching territory.

Moderate Calcium Deposits

For deposits that do not respond to chemistry adjustment, use a calcium-specific descaling product. These are typically acid-based solutions that you apply directly to the deposits at the waterline or on submerged surfaces. For waterline scale, apply the product with a scrub pad while the water level is lowered 2 to 3 inches below the scale line. For submerged deposits, products designed to disperse in the water and dissolve calcium over several treatment cycles are available. Follow the manufacturer instructions for dosing and reapplication.

Metal Stains (Iron, Copper, Manganese)

Metal stains on plaster are commonly mistaken for calcium issues but require different treatment. Iron stains appear rust-brown or red. Copper stains appear blue-green or teal. Manganese stains appear purple or black. The diagnostic test: apply a small amount of ascorbic acid (vitamin C) directly to the stain. If the stain lightens or disappears, it is a metal stain, not calcium. Treat metal stains by adding a sequestering agent to bind the metals in solution, then filter them out. For severe metal staining, a full ascorbic acid treatment (lowering chlorine to zero, adding 1 pound of ascorbic acid per 10,000 gallons, then sequestering) can remove stains from the entire pool surface.

How Does CYA Affect Plaster Pool Chemistry?

Cyanuric acid (CYA) has two significant effects on plaster pool chemistry that most service technicians underestimate. First, it binds with chlorine, reducing the active hypochlorous acid available for sanitation. Second, it affects the LSI calculation by contributing to total alkalinity, which can mask the true water balance condition.

CYA and Chlorine Effectiveness

CYA protects chlorine from UV degradation, which is valuable in outdoor pools. But it also reduces chlorine killing power by binding with free chlorine to form chloroisocyanurates, which are less effective sanitizers. At 30 ppm CYA, approximately 97% of the free chlorine is bound as chloroisocyanurate. At 80 ppm CYA, over 99% is bound. This means the minimum effective free chlorine level increases as CYA increases. The general rule is to maintain free chlorine at approximately 5% of the CYA level. At 50 ppm CYA, that means 2.5 ppm free chlorine minimum. At 80 ppm CYA, you need 4 ppm free chlorine to achieve the same sanitizing effectiveness.

CYA and the LSI Calculation

CYA contributes to what is called cyanurate alkalinity. Approximately one-third of the CYA concentration acts as alkalinity in the water. This means your total alkalinity test reading includes this CYA contribution, which overstates the carbonate alkalinity that actually drives LSI. To get an accurate LSI calculation, subtract one-third of your CYA level from the total alkalinity reading before plugging it into the LSI formula. Example: if your total alkalinity reads 100 ppm and CYA is 60 ppm, your adjusted alkalinity for LSI purposes is 100 minus 20 (one-third of 60) = 80 ppm.

Why This Matters for Plaster

If you calculate LSI using unadjusted total alkalinity in a pool with high CYA, you will overestimate the LSI value. The water may appear balanced or slightly scaling based on your calculation, when in reality the carbonate alkalinity is lower than you think and the water is actually closer to etching. This is how pools with "good" test numbers develop etching over time. The CYA-adjusted alkalinity tells the true story. For plaster pools, keep CYA at 30 to 50 ppm maximum and always adjust alkalinity for CYA when calculating LSI.

What Is the Best Maintenance Protocol for Plaster Longevity?

Plaster surfaces last 10 to 15 years on average, but chemistry-conscious service can extend that to 15 to 20 years. The protocol is not complicated, but it requires consistent attention to LSI and proactive management of the factors that cause etching and scaling.

Weekly Protocol

• Test and adjust pH to 7.4-7.6 at every visit (this is the highest-impact LSI action)
• Test free chlorine and maintain at 5% of CYA level minimum
• Brush walls and floor with a stainless steel brush to prevent calcium buildup and algae attachment
• Inspect the plaster surface for new staining, roughness, or discoloration
• Check and clean skimmer and pump baskets, backwash filter as needed

Monthly Protocol

• Full chemistry panel: pH, free chlorine, combined chlorine, total alkalinity, calcium hardness, CYA, temperature
• Calculate LSI using CYA-adjusted alkalinity and verify it falls between -0.3 and +0.3
• Adjust alkalinity and calcium hardness if LSI is trending out of range
• Inspect tile line for scaling and treat with appropriate cleaner if needed
• Check equipment for calcium buildup (heat exchanger, salt cell, flow sensors)

Seasonal Adjustments

In warm climates, summer water temperatures push LSI positive. Lower total alkalinity by 10 to 20 ppm during peak summer to compensate. In fall and winter, as temperatures drop, LSI shifts negative. Raise alkalinity back to the standard range and consider a slight calcium hardness increase if it has drifted low. These seasonal adjustments are the mark of a professional service provider and the primary reason plaster lasts longer under professional care than DIY maintenance.

Frequently Asked Questions

Sources & References

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