Symptom Library
Fiberglass Pool Blisters: Why Bubbles Form and How to Cure Them for Good
Bumps and bubbles on your pool's surface are osmotic blisters, pockets of pressurized fluid forming behind the gel coat. They're a chemistry problem, they multiply, and popping them makes everything worse. Here's what's actually happening under there.
Blisters on a fiberglass pool are osmotic: water molecules slowly pass through the gel coat, dissolve water-soluble compounds inside the laminate, and the resulting solution pulls in more water by osmosis. Pressure builds and the gel coat lifts into a bubble. Because the cause is water plus the laminate's own chemistry, not a scratch or a stain, blisters multiply over time, and the only lasting cure is opening them, drying the shell, rebuilding, and sealing the pool behind a new gel coat barrier.
What Blisters Are and Why They Form
Gel coat looks waterproof, but at the molecular level it's semi-permeable, water molecules can migrate through it very slowly. That's normal and usually harmless. Blisters happen when those molecules find something to react with on the other side.
Inside every fiberglass laminate are trace water-soluble compounds: bits of uncured resin, leftover glycols and binders from manufacturing, tiny voids and dry spots in the glass. When migrating water reaches them, it dissolves them into a concentrated solution. And concentrated solutions separated from fresh water by a semi-permeable membrane do one thing: they draw more water across to dilute themselves. That's osmosis, the same force that plumps a raisin in water.
The fluid pocket grows, pressure builds, and the gel coat, the weakest link, lifts away from the laminate into a dome. That fluid, by the way, is not pool water: it's typically acidic, often with a sharp styrene-like smell when a blister is opened. That smell is the fingerprint of an osmotic blister.
Why some pools and not others? Manufacturing quality (how completely the resin cured, how well the glass was wetted out), gel coat thickness, water temperature (warm water accelerates permeation), aggressive chemistry that degrades the gel coat, and plain age. A thinning, oxidized gel coat lets water through faster, which is why blisters often show up alongside chalking and cobalt spotting on the same aging shell.
How to Diagnose Blisters Yourself
Feel for domes
Run your hand slowly over suspect areas, floors and lower walls first, since they stay wettest and warmest. Osmotic blisters are smooth, rounded bumps, anywhere from pinhead to coin-sized, that clearly stand proud of the surface.
Rule out the impostors
Calcium nodules feel gritty, crusty, and white, that's scale, a different diagnosis. A single large soft spot that flexes under your palm is delamination, not a blister field. Blisters are smooth, discrete, and usually plural.
Press gently
A developed blister feels slightly springy, there's fluid under it. Don't push hard and don't puncture. You're confirming, not treating.
Count and map
Photograph the affected zones with something for scale and note the date. Recheck every month or two. New bumps appearing, or old ones growing, confirms active osmosis. A handful of static blisters is a contained problem; a spreading field is a surface-wide one.
Why You Should Never Pop Them
It's tempting. Don't. Puncturing a blister underwater doesn't relieve the problem, it connects the laminate directly to the pool. Water floods into the opened pocket and wicks along the glass fibers, spreading saturation deeper and wider than osmosis ever managed. The acidic blister fluid escapes into your water, the raw laminate starts absorbing, and you've converted a sealed chemistry problem into an open structural one. Popped blisters also can't just be dabbed with putty: underwater patch products won't bond to wet, contaminated laminate, so the "fix" traps water inside the shell.
What Happens If You Ignore Blisters
Osmosis doesn't take seasons off. Here's the usual arc:
- They multiply. The same laminate chemistry exists across the whole shell, so wherever water permeates next, new blisters form. What starts as a patch on the deep-end floor becomes a field.
- They merge and delaminate. Growing blisters connect underneath, and the gel coat separates from the laminate in sheets rather than domes. At that point spot repair stops being an option.
- They rupture on their own. Swimmer traffic and pressure eventually break blisters open, with all the water-intrusion consequences of popping them, minus your consent.
- The laminate weakens. Long-term water saturation attacks the resin-to-glass bond (hydrolysis), softening the structure itself. A blistered pool caught early needs surface restoration; one ignored for years can need structural rebuild too.
Proper blister repair is a sequence, and every step exists for a reason. Drain the pool safely (shell-float precautions in place). Open and grind out every blister down to sound, clean laminate, not just the visible domes, since neighboring gel coat is often already lifting. Let the shell dry thoroughly; sealing moisture inside the laminate just schedules the next blister crop. Rebuild the excavated pockets with resin and glass, fair the surface smooth, then spray the entire pool with factory-grade marine gel coat to restore a fresh, full-thickness water barrier everywhere, because the old, permeable gel coat is what let this happen. That's our blister repair service, typically paired with full resurfacing, and it's backed by a written warranty. Painting over blisters, for the record, seals acidic fluid under a coating that will bubble right off. We don't paint pools.
Can You DIY This?
Diagnosis and monitoring, absolutely. The feel test, the impostor checks, and a photo log cost nothing and genuinely help whoever treats the pool.
Repair, this is honestly one of the least DIY-able jobs in the symptom library. It stacks every hard thing at once: draining a fiberglass shell without floating it, grinding into laminate, judging when the shell is actually dry, wet-layup fiberglass work, and spraying catalyzed gel coat across an entire pool so it cures uniform and bonded. Miss the drying step, the one DIYers most often rush, and the blisters return under your brand-new surface. If you've confirmed blisters, send us photos: a specialist will gauge how far along the osmosis is and give you a straight answer and a real estimate within one business day, free.
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Straight Answers
Blister questions, answered
What causes blisters in a fiberglass pool?
Osmosis. Water molecules slowly permeate the gel coat and dissolve water-soluble compounds inside the laminate. The concentrated solution draws in more water, pressure builds, and the gel coat lifts into a bubble. Manufacturing quality, gel coat age and thickness, and warm or aggressive water all influence how soon it starts.
Should I pop a blister on my pool's surface?
No. Puncturing a blister underwater lets pool water flood the laminate and wick along the glass fibers, spreading damage deeper than the blister ever did. It also releases the acidic fluid inside and leaves raw laminate that no underwater patch will bond to. Confirm, photograph, and leave it sealed.
Will pool blisters go away on their own?
No. The osmotic driver, water plus soluble compounds in the laminate, doesn't resolve itself. Blisters grow, multiply, merge, and eventually rupture. The only lasting fix is opening them, drying the shell, rebuilding, and restoring a full-thickness gel coat barrier.
Can you paint over fiberglass pool blisters?
You can, and it fails fast. Paint traps the pressurized, acidic blister fluid under a coating that isn't bonded to the shell, so the blisters simply lift the paint next. Proper repair removes the damaged material and reseals with marine-grade gel coat, the surface the pool was built with.
How much does fiberglass pool blister repair cost?
It depends on how many blisters, how deep the saturation goes, and whether the whole surface needs recoating, which it usually does once blistering is widespread. Most full treatments fall in line with resurfacing costs. See the repair cost guide or send photos for a free estimate.
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