The short version
- An acid wash strips a thin layer of plaster to expose a fresh surface, so every wash uses up part of the finish.
- There is no fixed schedule. A plaster pool with well-balanced water may never need a routine wash; most washes correct a problem.
- Identify the stain first. Metal stains, leaf stains, and scale each have a gentler fix that works with the water still in the pool.
- Draining an inground pool is the riskiest part of the job, especially after heavy rain, because groundwater can push an empty shell upward.
The short answer: as rarely as you can
You should acid wash a plaster pool only when stains or scale cover large areas and gentler treatments have failed. It is a corrective job, not routine maintenance, and a pool with steady chemistry can go many years without one. You will see "every five years or so" quoted as a rule of thumb. Read it as the most often a pool with recurring problems might need one, not as a calendar reminder.
The reason is simple. Acid works by dissolving the outer skin of the plaster, taking the stain or scale with it and leaving fresher material underneath. Plaster has a finite thickness, so each wash brings the pool a step closer to resurfacing.
Repeated washes also tend to leave the surface rougher. A rough finish catches dirt, algae, and minerals more easily, which can start a cycle where the pool looks dingy sooner and seems to need another wash. Breaking that cycle starts with the water, not the acid.
Why pools around Houston scale and stain
How much calcium your fill water carries depends on where it comes from. Houston's main system draws about 90 percent of its water from lakes and rivers and the rest from deep wells, and its 2025 water quality report put average total hardness around 135 ppm. Sugar Land blends groundwater with Brazos River and Oyster Creek water, and its main system can run as high as about 280 ppm. Your city's annual water quality report lists the range for your system, which is a useful number to know before you blame the pool.
Heat makes it worse over time. Houston Intercontinental averages about 111 days a year at 90°F or above, and when pool water evaporates the minerals stay behind. Every top-off adds more, so calcium hardness creeps up season after season.
Whether that calcium stays dissolved depends on balance. Pool chemists use a saturation index that combines pH, alkalinity, calcium, temperature, and dissolved solids. When the index runs positive, calcium carbonate falls out of the water as scale on tile and plaster, and warm summer water pushes it higher. When it runs negative, the water pulls calcium out of the plaster instead and etches it. Some salt chlorinator manuals spell out a target of -0.2 to +0.2, with zero as ideal, and that is a sensible goal for any plaster pool.
- Scale: gray or white, crusty, and rough, often heaviest at the waterline, on steps, and near returns.
- Organic stains: green, brown, or tea-colored spots under leaves, berries, or debris that sat on the floor.
- Metal stains: brown, rust, blue-green, or gray patches, often from metals in fill water or from corroding equipment when the pH has run low.
Signs it may be time for an acid wash
These point toward a full acid wash, especially when more than one shows up at once:
- Scale across the floor and walls that feels like sandpaper and does not move with a brush.
- Widespread stains that stay put after you have identified them and treated them chemically.
- Blotchy discoloration left behind after a long-neglected green pool, even once the water is clear and balanced.
- A dull, dingy look over the whole surface while your water tests in range.
Signs it is not time yet
A lot of pools get acid washed for problems that have a smaller fix. Hold off if what you see matches one of these:
- A white or gray ring only at the waterline. That is tile scale, a separate job that does not require draining. See pool tile cleaning.
- A handful of isolated stains. Spot treatments can usually handle these with the water in.
- Leaf and debris stains. These often fade once chlorine is raised and the debris stops landing.
- Fresh plaster that is still curing. New plaster sheds dust and needs a careful startup, not acid.
- A vinyl liner or fiberglass pool. Acid washing is for plaster and similar cement-based finishes; acid can damage a liner or gelcoat.
Identify the stain before you treat it
Two quick tests tell you a lot. Hold a vitamin C (ascorbic acid) tablet against the stain for a short while. If the spot lightens, you are most likely looking at a metal stain. If a small amount of granular chlorine placed on the spot lightens it instead, the stain is organic. Brush the chlorine away once you have your answer rather than leaving it sitting on the plaster.
If neither changes it and the surface feels rough, you are probably dealing with scale or a stain set deep in the plaster. Write down what you tried and what happened. That record saves time and money whoever ends up treating the pool.
Gentler fixes to try first
Most of these work with the pool full, which avoids every draining risk below.
- Chemical stain treatment: ascorbic acid based products lift many metal stains, and a metal sequestrant afterward helps keep the metals dissolved so they do not redeposit. Follow the directions closely, since most require adjusting chlorine and pH first.
- Balance correction: bringing pH, alkalinity, and the saturation index back near zero stops new scale from forming and can soften light, fresh deposits. It will not dissolve thick scale.
- Waterline and tile cleaning: scale on tile is removed separately with scale removers, careful hand work, or fine media blasting. Glass tile scratches easily, so method matters.
- Spot treatment: a few stains can often be treated individually without draining.
- Resurfacing: if the plaster is thin, rough, hollow-sounding, or already washed several times, money spent on another wash is better put toward new plaster. See pool renovations.
What draining a pool risks, and how to reduce it
An inground pool's water weight helps hold the shell down. Empty it while the ground around it is saturated, and groundwater pressure can lift or crack the shell. The risk is highest where the water table is high and right after several days of heavy rain.
That matters here. June is the wettest month on average at Houston Intercontinental, and hurricane season runs from June 1 through November 30. Many inground pools have a hydrostatic relief valve in the main drain that lets groundwater in rather than letting it push the shell up, but you should not assume yours has one or that it still works.
Exposed plaster also suffers in hot sun. Left dry too long, it can blister or crack, so the goal is to drain, wash, and refill in one tight window. The equipment needs protecting too: if the circulation pump kicks on after the water drops below the skimmer, it pulls air and can damage itself.
- Pick a dry stretch of weather and avoid draining after heavy rain or with a tropical system in the forecast.
- Turn off the pump, heater, salt system, and any timer or automation schedule before the level drops, and drain with a submersible pump instead of the pool's own pump.
- Send the water where your city allows. Many ask for a sewer cleanout rather than a storm drain, so check the local rules first.
- Neutralize spent acid before pumping it out, and wear goggles, chemical-resistant gloves and boots, long sleeves, and a respirator rated for acid fumes. Always add acid to water, never water to acid.
- Keep children and pets out of the yard while the pool is empty. A drained pool is a serious fall hazard.
- Refill promptly, then rebalance carefully, because freshly exposed plaster is more reactive for a while.
