Blistering is when bubbles form under a coating and push it up from the surface. It’s usually caused by moisture vapor or trapped gas building pressure beneath the film. Salts in the concrete can also contribute. Blisters often grow and eventually break open.
Why Blistering Matters for Your Floor
A blister is a spot where the coating has already let go of the concrete. Think of a bubble under a phone screen protector: it looks small, but nothing is holding that patch down anymore. Once a blister breaks, water, dirt, and road salt can get under the edges and the peeling spreads.
On a garage or basement floor, blistering is usually a sign of a problem in the slab, not just in the coating. That’s why patching the bubbles rarely ends the story. If the cause is still there, new blisters tend to show up next to the old ones.
What Causes Blistering on Concrete and Epoxy Floors
Most coating blisters trace back to one of two things:
- Moisture coming up through the slab. Concrete holds water and slowly gives off vapor. When that vapor gets trapped under a dense coating like epoxy, it can cause bubbling, softening, and delamination. Basements and slabs without a vapor barrier underneath are the usual suspects.
- Air escaping from the concrete. Called outgassing, this happens when air in the slab pushes up through the wet coating, often when the slab warms up during the job. It leaves bubbles, craters, or pinholes, though it typically doesn’t weaken the bond the way moisture does.
A third type, called osmotic blistering, happens when moisture is drawn toward salts or other dissolved material at the bond line. They tend to show up in slabs with no vapor barrier or with salts in the concrete.
What to Ask Your Contractor About Blistering
Most blistering is preventable with good prep and testing. Before you sign, ask:
- How will you test the slab for moisture? Moisture testing before coating is the single best way to avoid blisters.
- What happens if the slab tests high? The usual answer is a moisture-blocking primer under the coating.
- How will you handle air bubbles? Ask your installer how they’ll watch for and deal with outgassing during the job.
- What does the warranty cover? Ask your installer how the warranty handles moisture problems.
If you already have blisters, a contractor will typically grind out the failed spots, fix the moisture issue if there is one, then patch and recoat. Widespread blistering usually means removing the coating and starting over. Our guide to concrete floor coatings covers what a full system looks like.
Blistering FAQ
What causes epoxy to bubble up?
Bubbles in an epoxy floor usually come from air or moisture escaping the concrete. Air bubbles show up during application and cure, while moisture blisters can appear weeks or months later. A moisture test before coating helps tell which one you’re dealing with.
What would cause an epoxy floor to bubble after it’s cured?
Late blisters on a cured floor almost always point to moisture vapor rising through the slab. Pressure builds under the coating until it lifts. Basements and older garages without a vapor barrier see it most.
Can you repair a damaged epoxy floor with blisters?
Yes, if the blistering is limited to a few spots. The contractor grinds out the blisters, patches, and recoats the area. If blisters cover much of the floor, the coating usually has to come off so the moisture problem can be fixed first.
Will blisters in a floor coating go away on their own?
No. A blister means the coating has lost its bond in that spot, and it won’t reattach. Blisters usually grow or break open over time, so it’s better to repair them early.