Gunite Pool Maintenance: What Makes It Different

"Gunite pool" is the most common way people describe an in-ground concrete pool, but the gunite itself never touches your skin or your water. What you actually swim in, brush, and test against is a plaster or aggregate finish troweled over that structural shell, and that finish has a real chemical relationship with your water. Understanding that distinction is the difference between maintaining a gunite pool well and slowly wearing one out.

Gunite is the shell, plaster is the surface

Gunite (and its close cousin, shotcrete) is a sprayed concrete mix applied over a rebar frame to form the structural walls and floor of the pool. It's rough, gray, and porous, essentially the skeleton of the pool. Before the pool is ever filled, a finish coat goes over that shell: traditional white plaster (sometimes called marcite), plaster blended with quartz aggregate for extra durability, or a pebble aggregate finish for a more textured, mottled look. That finish coat is what you feel underfoot, what algae stains, and what your water chemistry acts on directly. When people talk about "gunite pool maintenance," they're almost always really talking about caring for this surface.

Why calcium hardness matters more on a gunite pool

Plaster and pebble finishes are cementitious, meaning calcium is part of their actual structure, not just something dissolved in the water sitting on top of them. That creates a two-way relationship a vinyl liner or fiberglass shell simply doesn't have. If your calcium hardness runs low, the water becomes aggressive and pulls calcium out of the surface itself, leaving it rough, chalky, and pitted over time. If calcium hardness runs high (especially alongside high pH or high alkalinity), the water does the opposite and deposits scale onto the surface as cloudy gray or white buildup. Vinyl and fiberglass pools mostly shrug off calcium swings; a gunite pool's surface is directly downstream of them.

This is why pool professionals often look at the Langelier Saturation Index (LSI) on plaster and gunite pools rather than judging calcium hardness in isolation. LSI combines pH, calcium hardness, total alkalinity, and water temperature into a single balance score: roughly below -0.3 is considered corrosive to plaster and metal, -0.3 to 0.3 is the balanced range, and above 0.3 tends to be scale-forming. Skimmer's LSI calculator can combine your latest readings into that single read so you're not eyeballing four numbers separately.

These are the same balanced-water ranges that apply to any pool, but the consequences of drifting outside them are more visible and more permanent on a plaster or pebble surface. A vinyl liner pool that runs a little low on calcium hardness for a few weeks rarely shows any lasting damage. A gunite pool doing the same thing is slowly etching its own surface, it just won't be obvious until months later.

The plaster resurfacing cycle

Even a well-maintained plaster or pebble surface wears down eventually. That's expected, not a sign you did something wrong. Many pool professionals estimate a standard white plaster finish typically lasts somewhere in the range of 7 to 15 years before it needs resurfacing, while quartz-blended and pebble-aggregate finishes often run longer, sometimes 15 to 20-plus years, thanks to their harder aggregate content. Actual lifespan varies a lot based on water balance discipline, climate, how heavily the pool is used, and how well the original plaster job was applied and cured. Chronic low calcium hardness or corrosive LSI readings tend to shorten that timeline noticeably; consistent water balance stretches it.

• A rough, sandpaper-like texture underfoot that doesn't smooth out even after brushing (a sign of etching).
• Dark, blotchy staining or discoloration that doesn't respond to normal brushing or spot treatment.
• Small pits, cracks, or chips that expose the gray gunite shell underneath the finish coat.
• Persistent cloudiness that clears after balancing the water but keeps coming back, sometimes fine plaster particles settling on the floor.
• Rough calcium nodules or scale buildup on the surface or steps that brushing alone can't remove.

A maintenance rhythm that protects the surface

Brushing matters more on a gunite pool than on a smooth vinyl or fiberglass surface, because plaster and pebble finishes are porous and give algae and stains something to grip. A weekly brush of the walls, floor, and steps helps prevent both staining and early calcium scale from bonding to the surface. It's also worth avoiding dropping chlorine tablets or granular shock directly onto plaster and letting them sit in one spot; the concentrated chemical can bleach or locally etch the finish. A floating feeder or broadcasting granular product evenly across the surface spreads that exposure out instead of concentrating it in one place.

It also helps to test more often than "whenever something looks off." Because plaster damage from imbalanced water shows up gradually, sometimes over months, a weekly test is what catches a slow drift in calcium hardness or pH before it becomes visible etching or scale. This is where a running log actually earns its keep: a single reading tells you today's number, but a trend line tells you whether your calcium hardness has been creeping down for six weeks straight, which is exactly the kind of pattern that's easy to miss test to test and easy to catch once it's logged. Skimmer logs your pool's surface type alongside every water test, so you can look back at trends over time instead of just whether last week's reading passed or failed.