Why Does My Epoxy Floor Have Pinholes?
A pinhole is a bubble that broke before the film could flow back together. That one sentence explains why pinholes and bubbles share their causes but need completely different fixes: one is about the gas, the other is about whether the coating could still heal when the gas got out.
A pinhole is a bubble that broke before the film could flow back together. The gas is usually the same gas that causes bubbling, air coming out of a warming slab or air you whipped in yourself, but the thing that decides whether you get a dome or a hole is the film: how thick it is, how fast it is gelling, and whether anything sealed the substrate first. Prime porous concrete, mix slowly, and lay the film at the build the sheet specifies rather than the build you get by spreading it further.
Most likely causes
Air out of an unsealed slab
- Every pore in bare concrete holds air, and it expands as the slab warms
- Worst on the first coat over open, freshly ground substrate
- Fix: prime porous slabs, so the pores are sealed at a thin build where a pinhole costs nothing
Air whipped in at the mixer
- A drill run too fast, or a paddle lifted so it breaks the surface
- The air goes in as a froth and comes out through the film
- Fix: fold rather than beat, keep the paddle submerged, and let the mix stand a minute where pot life allows
Film too thin to heal
- A bubble that breaks in a generous film closes again; in a thin one it stays open
- Usually a spread rate problem rather than a decision anyone made
- Fix: check the rate you are achieving against the rate the target build needs
Solvent pop
- The surface skins over before the solvent underneath has left, and it punches out
- Driven by sun, a hot slab, forced air, or too heavy a pass on a solvent borne product
- Fix: slow the surface down rather than the job up. Shade it, kill the draught, thin the pass
Telling pinholes from bubbles and fisheyes
These three defects are all round, all small, and all get called bubbles on site. They have three different causes and three different fixes, so it is worth thirty seconds on your knees with a light held low across the floor.
Is it raised or is it a hole?
- Bubble: a closed dome. The gas is still under the film
- Pinhole: a puncture through an otherwise flat film. The gas got out
- Fisheye: neither. The coating pulled back from a spot and left a rim
What is at the centre?
- Pinhole: a hole, with coating right up to its edge
- Fisheye: bare slab or the coat below, often with a speck in the middle
- That difference is the one that tells you whether to change your technique or go looking for contamination
Does it come back in the same place?
- Gas defects move. The air that caused them has gone, so a recoat lands somewhere else
- Contamination repeats. It is still on the slab, so the next coat does the same thing in the same spots
- The cheapest test you have, and it settles the argument
Check the film you are actually laying
Measured spread mode takes the area you covered and the product you used and returns the dry film that rate produces. Run it on the first bay, not the last one.
Returns 6.4 mil of dry film, against a 10 mil target
Open the live calculator →Worked example: the crew that was running too far
A specification calls for 10 mil of dry film from a 100% solids product. To land that, the crew has to be spreading at 160.4 ft²/gal, because coverage and thickness are the same fact stated two ways.
On the first bay they cover 250 ft² with a gallon. That feels efficient, and nobody stops. But at 250 ft²/gal the film is 6.4 mil, which is a little under two thirds of what was specified. The floor is not just thin against the spec; it now has far less material available to flow back over any bubble that breaks at the surface, which is exactly the condition that turns escaping air into a permanent hole rather than a mark nobody ever sees.
This is why pinholing and a thin floor so often arrive together, and why the two get diagnosed as separate problems. Caught on the first bay it is a conversation about pace. Caught at the end it is a conversation about a recoat, and by then the pinholes are cured in. Work yours out in the film thickness calculator before the second bay goes down.
How to repair a pinholed floor
The instinct is to put another coat over the top, and it is usually the wrong move, because a coat laid over a hole tends to bridge it rather than fill it. Worse, if the slab is still releasing air, the new layer gets its own set of pinholes in the same places, which is how a floor gets coated three times and still fails.
The sequence that works is to abrade first. Sanding or a light grind takes the gloss off, opens the holes so material can get down into them, and gives the repair coat something to key to. Then work a coat into the surface rather than laying it on: on a badly pinholed floor a scratch coat or thin skim pulled tight with a squeegee fills the holes, and the finish coat goes over a flat surface. Give the substrate a chance to stop gassing before you start, which usually means coating on a falling temperature rather than a rising one.
How to prevent it on the next job
Before you mix
- Prime porous or freshly ground concrete rather than going straight to a build coat
- Check the temperature curve. A slab that is about to warm will push air all afternoon
- Read the sheet for the film build and the recoat window, not just the coverage
While you work
- Mix slowly with the paddle submerged, and scrape the sides and base
- Use a spiked roller where the product calls for one, which is what releases entrained air
- Check your spread rate on the first bay and hold the crew to it
Most of that is free. The one thing that costs money is the primer, and it is reliably cheaper than the coat you have to grind off. Conditions are worth checking too, since a rising slab is the single most common driver: the dew point calculator tells you what the air and the slab are doing before the first pail is opened, and the data sheet guide covers where the film build and recoat window come from and what conditions they assume.
Frequently Asked Questions
What is the difference between pinholes and bubbles?
They start the same way and end differently. Gas rises through the wet film and forms a bubble. If the film gels while the bubble is still domed over, you get a bubble. If the bubble breaks at the surface and the coating is still fluid enough to flow back, the hole closes and you never know it happened. A pinhole is the third case: the bubble broke, and the film was too stiff or too thin to close over the hole before it set.
That is why they need different fixes. Bubbling is solved by removing the gas source, usually moisture or a warming slab. Pinholing is often the same gas, but the thing to change is the film: how thick it is, how fast it is gelling, and how much air you put into it yourself.
Is a pinhole just cosmetic, or does it matter?
It depends whether it goes all the way through. A shallow surface pit is cosmetic and will collect dirt. A pinhole that penetrates to the substrate is a hole in the barrier, and on a floor specified for chemical resistance, hygiene or moisture protection that is a functional failure, not a blemish.
On a garage or a workshop the practical concern is dirt and staining. On a food, pharmaceutical or wet process floor it is the whole point of the coating, so pinholes get repaired rather than accepted.
Can I just topcoat over pinholes?
Only if you know they have stopped gassing, and often they have not. Coating over a pinhole that is still releasing air from the slab gives you a fresh set of pinholes in the new layer, usually in the same places.
The reliable sequence is to abrade the surface, which also opens the holes so a coat can get into them rather than bridging over them, then apply a coat at the specified film build. On a badly pinholed floor a scratch coat or a thin skim worked into the surface with a squeegee fills the holes before the finish coat goes on.
Why do pinholes show up more on the first coat?
Because the first coat is the one meeting bare, porous concrete. Every pore in the slab holds air, and that air expands and escapes as the slab warms or as the resin displaces it. A primer exists partly to deal with this: it seals the pores at a thin build where the consequences are small, so the build coat is going over sealed substrate rather than an open one.
Skipping the primer on a porous slab and going straight to a build coat is one of the more common routes to a pinholed floor.
Does mixing speed really cause pinholes?
Yes, and it is one of the easiest to fix. A drill run too fast, or a paddle lifted so it breaks the surface, whips air into the resin. That air has to come out somewhere, and it comes out through the film.
Mix slowly enough that you are folding rather than beating, keep the paddle submerged, scrape the sides and base so nothing unmixed rides along, and where the pot life allows it let the mixed material stand for a minute before pouring. Do not extend that stand time on a fast product: pot life is the harder constraint.
What is solvent pop, and is it the same thing?
It looks similar and it is worth separating because the fix is different. Solvent pop happens when the surface of a coating skins over before the solvent underneath has finished leaving. The trapped solvent then forces its way out through the skin, leaving holes that look like pinholes.
It is driven by anything that dries the surface faster than the body: direct sun, a hot slab, forced air over the film, or too heavy a build in one pass on a solvent-borne product. The tell is that it appears on solvent-containing products rather than 100% solids ones, and it tends to be worse where the film is thickest. The fix is to slow the surface down, not to speed the job up.