Calcium Nodules on Pool Plaster in Los Angeles: Why They Grow

Most people find the first one with a bare foot. A rough little mound on a step, about the size of a pea, sometimes with a brown dot in the middle, and brushing does nothing to it. That is a calcium nodule, which pool techs also call a calcium volcano. The white crust that hard water leaves along the tile is a separate problem, covered on our pool chemistry page. This page is about the bumps your test kit had no part in.

How a nodule grows

Plaster is cement, and cement holds calcium hydroxide in its pores. Where the plaster has cracked, or has come loose from the shell behind it, pool water gets in and dissolves some of that calcium. Then it seeps back out through the same opening. At the mouth of the crack it meets the pool water and the carbon dioxide in it, and the dissolved calcium turns into calcium carbonate: hard, white and insoluble at normal pH. Layer after layer, a cone builds up around the hole. Hence the volcano.

The CPO training material that tops the search for this names two causes: cracking from plaster that dried out too fast, and bond failure, also called delamination, which shows up mostly in pools that were replastered. The onBalance pool guides put nodules at a quarter to half an inch across, with the size set by how large the loose patch behind them is. A brown or rust center means the water has reached metal, often a tie wire or the rebar in the shell, and that is worth taking seriously.

Nodule or scale: check before you scrape

• A nodule is one raised point, hard as stone, often on a hairline crack. Scale is a flat crust or rough film spread over an area, thickest at the waterline.
• Nodules come back in the same spot after you take them off. Scale comes back wherever the water runs high.
• Your readings say nothing about nodules. Scale follows calcium hardness above 400 ppm and a pH that sits high.
• Tap around the bump with the end of your pole. A dull, hollow sound means the plaster has let go of the shell there.

Crust on the waterline tile is a tile cleaning job and comes off the tile itself. Flat brown or rust spots that sit level with the plaster are stains, and the two spot tests in our stain removal guide tell you which kind.

Fine cracks in a pattern like dry mud are called crazing, and in Los Angeles they are the usual starting point. Plaster cracks this way when it dries in the sun, on the day it goes on or years later when a pool stands empty through an August afternoon. It is why a drained shell gets refilled the same day, as our guide on draining a pool explains, and why new plaster is brushed daily through its first month on the plaster start-up.

Taking them off yourself

A few nodules on firm plaster can come off by hand. It takes the sharp edge off a step and makes the pool look right again. Expect them back, though, wherever water still gets in behind the plaster.

1. Rub the bump down underwater with a pumice stone or a wet sanding block. Stop when it is level with the plaster around it; going deeper scars the finish.
2. For a stubborn one above the waterline, or on a step you have exposed by lowering the water, brush on a little muriatic acid, wait a minute and rinse it off well. Wear gloves and eye protection, and keep that plaster wet so it does not dry in the sun.
3. Close the crack. A two-part underwater epoxy putty or a plaster patch kit pressed into the opening slows the next nodule down. On a crack that keeps moving, it will not hold for long.
4. Test pH the next day. Acid that reached the pool pulls it down.

When it needs a patch or a new finish

A handful of nodules on one step, with smooth plaster around them that sounds solid when you tap it, is local damage. A patch cuts out the loose section and fills it with new plaster. It stops water reaching the shell and buys more seasons, though it never disappears: new plaster against plaster that has had years of sun always shows.

Nodules spread over the floor, hollow sounds in several places, rust in the centers and plaster that has gone rough mean the bond is failing over a wide area. Patch one spot and the water finds the next. That is where resurfacing starts to make sense, and with delamination the old coat has to come out rather than be plastered over. Fifteen to twenty years is the usual life of a finish, so a pool replastered three or four years ago that is already growing nodules points to how the plaster went on. Dig out the paperwork from that job before you pay for the next one.

Replastering anyway? Our Pebble Tec and plaster comparison covers how each finish ages in LA water.

What it costs

Doing it by hand costs a pumice stone, a tube of epoxy putty and an afternoon on the steps. A patch is priced by how much loose plaster has to be cut out, which nobody can tell from above the water, so it is quoted after someone taps the floor. A full resurface starts at $7,500, and most projects land between $10,000 and $20,000, as our resurfacing cost page sets out. After new plaster, the 28-day start-up is $595 for a pool up to 25,000 gallons.

We can usually tell nodules from scale from one picture taken straight down at the step, with the year the pool was last plastered. Send it here or ring (818) 740-6099; diagnosis is $0 inside our service area.