EpoxyGrindDIY guide
Epoxy Bubbles and Fisheyes: Causes and Fixes
Bubbles and fisheyes look similar but come from opposite causes — one is trapped air, the other is a surface contaminant repelling the coating.
Bubbles and fisheyes are two different defects that get lumped together because they both show up as small surface imperfections. Bubbles are trapped air or gas — either whipped into the mix during application, or pushed up from the concrete itself as it "outgasses" (releases air and vapor from its pores as ambient temperature rises after coating). Fisheyes are craters or dimples where the epoxy actively pulled away from a spot, almost always because of a contaminant — oil, wax, silicone, even skin oil from bare hands — that the coating can't wet out and bond to. Different cause, different fix.
Identify which one you have
Bubbles are typically small, roughly round, and often appear in clusters or a honeycomb pattern — they look like something was trapped underneath and pushing up. Fisheyes are craters with a distinct edge, like a drop of water on a waxed car hood — they look like the coating is pulling away from a specific spot rather than something pushing up through it.
Prevent bubbles from mixing: use the right RPM and paddle
Mix at a low RPM (roughly 300-500, check your product's spec) with a proper mixing paddle rather than a high-speed drill setting. Aggressive, fast mixing whips air directly into the batch — that trapped air has to go somewhere, and it often surfaces as bubbles during application.
Prevent bubbles from outgassing: watch temperature and don't rush the pour
Outgassing happens more in a slab where trapped air and vapor are trying to escape as temperature rises after you've sealed it under wet epoxy — this is more common on new or porous concrete, or when coating during a rising-temperature part of the day. If you suspect outgassing risk, coat during stable or falling temperatures rather than early morning with a hot afternoon ahead, and consider a properly-timed second coat rather than one very thick pour.
Use the right roller, and pour rather than dip
Dipping a roller into a bucket repeatedly can push air into the product. Pour epoxy onto the floor and roll it out from there instead. A mohair or foam roller cover designed for epoxy also introduces less air than a standard nap roller.
Prevent fisheyes: fully degrease and clean the slab before coating
This goes back to prep — any oil, grease, wax, or silicone-based product (including some concrete sealers or curing compounds from the original pour) left on the surface will cause the coating to pull away from that exact spot. See our concrete prep guide for the degreasing step, and avoid touching a freshly-ground, cleaned slab with bare hands before coating — skin oil is a real, if minor, contamination source.
Fix small bubbles or fisheyes while the coat is still wet
If you catch a bubble or fisheye while the coating is still workable, a light pass with a heat gun (held at a safe distance, moving continuously) can help release trapped air and encourage the coating to flow back into a fisheye crater. Don't linger in one spot — too much localized heat can scorch the coating.
Fix bubbles or fisheyes after the coat has cured
Once cured, lightly sand the affected spot with fine-grit sandpaper to remove the defect and rough up the surface for adhesion, clean thoroughly with an appropriate cleaner (avoiding anything that could reintroduce a contaminant), and apply a fresh, thin patch coat over the sanded area.
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Mistakes to avoid
- Mixing at too high an RPM — this is the single most common cause of bubbles that have nothing to do with the concrete itself.
- Coating over visible fingerprints, dust, or a spot you touched with bare hands after cleaning — skin oil alone is enough to cause a small fisheye.
- Coating during rapidly rising temperatures on a porous or new slab — this is when outgassing risk is highest and least controllable.
- Panicking and adding a thick second coat over bubbles or fisheyes without fixing the root cause first — a thicker coat over an uncorrected contamination spot usually just creates a bigger fisheye.
FAQ
Are a few small bubbles normal, or does it mean I did something wrong?
A handful of very small, scattered bubbles in an otherwise sound coat is common even among experienced installers and usually isn't a structural problem — dense clusters or bubbles concentrated in one area point to a specific, fixable cause worth diagnosing.
Can I prevent outgassing entirely on a brand-new concrete pour?
Not entirely — new concrete continues to cure and release trapped air/vapor for weeks. Waiting the full recommended cure time (28+ days) before coating, and avoiding coating during a hot, rising-temperature part of the day, reduces the risk significantly but can't eliminate it completely on very new concrete.
Will sanding and patching a fisheye be visible afterward?
On a solid color coat, a well-blended patch is usually subtle but can be faintly visible under close inspection or raking light. On flake or metallic finishes, the texture and pattern help hide small patches more effectively.
Does a thicker epoxy system reduce bubbles and fisheyes?
Not directly — thickness (mils/solids content) affects durability, but bubbles and fisheyes are caused by mixing technique, temperature, and surface contamination, all of which matter regardless of how thick your system is.