Symptom Library
Black Spots in a Fiberglass Pool: What They Are and How to Fix Them
Black spots on a fiberglass pool surface come from one of three things: cobalt staining from inside the laminate, metal stains from your water, or black algae. Each one has a different fix, and only one of them can be treated with chemicals.
Black spots in a fiberglass pool are almost always one of three conditions: cobalt staining (a chemical reaction inside the fiberglass itself), metal staining (iron, copper, or manganese dropping out of the water), or black algae. Metal stains respond to a vitamin C test, algae responds to chlorine and brushing, and cobalt spots respond to nothing, because the stain is coming from below the surface. That last one is the diagnosis most pool owners have never heard of, and it's the one we treat most.
What Causes Black Spots in a Fiberglass Pool
Fiberglass pools are built from polyester resin cured with a catalyst system. Part of that system is a cobalt compound, a promoter that helps the resin harden at the factory. In most pools it does its job and stays put. But in some shells, especially as the gel coat ages and thins, that cobalt can migrate toward the surface and oxidize when it meets water and chlorine. The result is small blue-black or gray-black spots that look like they're on the surface but are actually coming from within the laminate.
That's cobalt staining, and it's the fiberglass-specific culprit. The other two suspects can hit any pool:
- Metal stains. Iron, copper, or manganese in your fill water, or copper from an old heat exchanger or algaecide, oxidizes and plates onto the surface. Iron runs brown-black; copper runs blue-green to black.
- Black algae. A stubborn organism that roots into rough or porous spots. On a fiberglass pool in good condition it's rare, because the gel coat is too smooth to grip. If you have black algae on fiberglass, the surface itself is usually degraded, which is its own symptom worth reading about.
How to Diagnose Black Spots Yourself
You can narrow this down in an afternoon with three simple tests. Do them in this order.
The scrub test
Brush a spot hard with a soft nylon pool brush (never wire on fiberglass). Black algae feels slightly raised and slimy, and brushing breaks its protective layer. Stains, metal or cobalt, are flat and smooth. If the spot is flush with the surface, it's a stain, not a growth.
The vitamin C test
Hold a plain ascorbic acid (vitamin C) tablet directly against the spot for 30-60 seconds, or rub it gently. If the spot lightens or disappears, you have a metal stain, most likely iron. Copper stains may need a citric-acid-based product instead, but the principle is the same: metal stains react to mild acid.
The chlorine test
Put a small amount of granular chlorine in a sock and hold it against the spot (or use a chlorine tablet with care). Black algae will lighten and die back. Metal stains may darken. Cobalt spots won't budge.
The tell that clinches it
If a spot fades when you sand or polish it lightly and then bleeds back in the same place weeks later, that's cobalt. The stain source is inside the laminate, so surface treatment only erases it temporarily. Recurring spots in fixed locations are the cobalt signature.
One more clue: pattern. Metal stains tend to appear broadly or follow water circulation, often after adding fill water or shocking the pool. Cobalt spots are discrete dots, frequently on the floor and lower walls, that show up one by one and slowly multiply as the gel coat wears thinner.
What Happens If You Ignore Black Spots
Depends on the diagnosis. Ignored black algae spreads and roots deeper into a degrading surface. Ignored metal staining keeps plating until the whole pool dulls and discolors, and the aggressive chemicals people use to fight it can etch the gel coat, making everything worse.
Ignored cobalt is the quiet one. The spots themselves don't leak or spread like algae. But cobalt staining is a symptom of a bigger condition: gel coat that has thinned or been compromised enough to let the chemistry underneath reach water. Pools with active cobalt spotting often develop osmotic blisters and heavy oxidation on the same timeline, because all three trace back to an aging gel coat barrier. The spots multiply, sanding stops keeping up, and eventually you own a polka-dot pool no chemical on the market can whiten.
Metal stains: ascorbic or citric acid treatment, then a metal sequestrant and better source-water management. Black algae: aggressive brushing and chlorination, plus fixing the rough surface that let it root. Cobalt spots: there is no chemical cure. The affected gel coat has to be ground out (excised) down to clean laminate, the area rebuilt, and the surface refinished with new marine-grade gel coat. For a pool with widespread cobalt spotting, spot repairs turn into a losing game of whack-a-mole, full gel coat resurfacing removes the stained layer everywhere at once and gives you a fresh 15-25 year barrier. Not paint. Paint over cobalt just gives the spots a new canvas to bleed through.
Can You DIY This?
Some of it, honestly, yes. The vitamin C test and a metal stain treatment are well within a pool owner's reach, follow the product directions, keep your pH in line, and use a sequestrant afterward. Brushing and shocking black algae is sweat equity, not skill.
Cobalt is different. The repair means draining the pool (which carries real risk on a fiberglass shell), grinding into the laminate, and spraying catalyzed gel coat that has to be mixed, applied, and cured correctly to bond. Get it wrong and you've added a patchwork of dull, mismatched repairs to a spotted pool. If your spots pass the "bleeds back after sanding" test, that's the point to send us photos, a specialist can usually confirm cobalt from good close-ups and tell you whether excision or full repair and resurfacing makes sense for your shell. The checkup is free and answered within one business day.
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Straight Answers
Black spot questions, answered
What are the black spots in my fiberglass pool that won't scrub off?
Flat black spots that survive scrubbing, chlorine, and stain removers, and return after sanding, are almost certainly cobalt staining. The cobalt compound used to cure the pool's resin has migrated and oxidized within the laminate, so the stain regenerates from below the surface.
Can chemicals remove cobalt spots from a fiberglass pool?
No. Stain removers and acids work on deposits sitting on the surface. Cobalt staining originates inside the fiberglass itself, so chemical treatment can lighten it temporarily at best. The lasting fix is removing the affected gel coat and refinishing with new gel coat.
How do I tell a metal stain from black algae?
Touch and test. Algae is slightly raised, feels organic, and lightens under direct chlorine contact. A metal stain is perfectly smooth and lightens when you hold a vitamin C tablet against it. If neither test moves the spot, think cobalt.
Will black spots damage my pool or just look bad?
Metal stains and algae are mostly cosmetic if handled promptly. Cobalt spotting is cosmetic too, but it signals a worn gel coat barrier, and pools that spot often blister and chalk on the same timeline. Treat it as an early warning, not just a blemish.
How much does it cost to fix cobalt spots?
A few isolated spots can be excised and patched for far less than a resurface. Widespread spotting usually points to full gel coat restoration, which for most residential pools runs into the five-figure range depending on size and condition. See our repair cost guide or send photos for a real number.
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