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Epoxy Troubleshooting - Bubbles, Pinholes, and Craters

By Jack Josephsen

Why do some epoxy users get bubbles, pinholes and craters in concrete coatings while others don't?

I guess the answer to that is part education, part habit and perhaps even part luck! In other words, those that don’t suffer know their causes, practice the right habits to prevent them and, in some cases, just have a bit of good old-fashioned luck.

Bubbles, pinholes, craters - education

The education part comes from understanding what causes bubbles, pinholes and craters in concrete coatings.

It all has something to do with the fact concrete is porous. Granted there are different degrees of porosity, but the reality is they are all porous on the scale we’re considering here. With a porous substrate, the voids are filled with air that expands when heated and contracts when cooled. When the air expands it pushes out of the slab through the path of least resistance, which in this case is the top, especially if we’ve “opened up” the slab through grinding or other mechanical means.

To get to the bottom of why these defects occur in a coating film, we need to consider what happens when a concrete coating is applied and air is being expelled in this manner. Whilst the coating is wet, the air being pushed through will blow a bubble - what happens after that point depends on how much air is coming through and whether the bubble pops or not. If the concrete coating has hardened sufficiently and the bubble doesn’t pop, you are often left with a nice complete bubble. If the bubble pops after the concrete coating has gelled and can’t flow anymore, it will leave a pinhole. Finally, if a large bubble pops after the concrete coating has gelled, it will leave a crater (sometimes with a loose “skin” attached).

A close-up photo of large, visible pinholes in a brown protective epoxy coating.

Bubbles, pinholes, craters - habits

The habit part relates to routinely doing whatever you can to prevent these defects from happening. Two very effective measures are: applying only on the cooling cycle of the slab, i.e. when the air is cooling and drawing in rather than expanding out, and, using an epoxy concrete sealer to block the voids before applying the concrete coating. The air will still be expanding and contracting, but with a sealed top the path of least resistance is no longer through the surface.

Just as a side note on sealing, be aware applying one thin film across the whole floor doesn’t guarantee all the voids are closed off. If some areas still appear dry then there could still be the potential for air to escape, so you might look to seal again or until the slab at least looks more even.

Bubbles, pinholes, craters - luck

Finally, the naive luck side of things refers to the small percentage of users that have no idea why bubbles, pinholes and craters happen, yet they seem to avoid them most of the time. They apply concrete coatings at all times of the day, onto slabs with all sorts of porosity, however they somehow get through largely unscathed. Some days they might get pinholes and other days they might not. Who knows?

While this might be frustrating for others that aren’t so lucky, rest assured it will catch up with them sooner or later! If you’ve only relied on luck in the past, I hope you now have the education and the habits to help you control your bubble, pinhole and crater destiny.

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