Epoxy troubleshooting - epoxy "hail damage"

What is epoxy “hail damage” and how is it linked to so-called Benard Cells?

The chances are you’ve already seen the effect of Benard Cells but didn’t understand what it was or how to overcome it. The effect can take on a few forms, but one of the most common is the hail damage appearance, which can leave epoxy films with lots of little dimples or dents and looking like a car that’s just driven through a nasty storm.

Tiny cells cause epoxy dimples

In very basic terms, Benard Cells are the result of the resins behaving like other liquids when it comes to convection currents - a pattern of flow that sees liquids rising as they warm and sink as they cool. As the film hardens the flow eventually stops and you can end up with an epoxy dimple in the middle of these cells, i.e. hail damage. It’s definitely not good news if you’re expecting a flat, glossy film on a high-end decorative floor!

A diagram showing why Benard cells occur and the effect they have on an epoxy film.

How can epoxy hail damage be stopped?

So, how can we prevent these epoxy dimples from ruining our projects?

Well, there are a few points to make about Benard Cells and epoxy hail damage that will help -

  1. They tend to be more of a problem in clear, low-viscosity resins.

  2. They tend to be worse with higher builds. From my experience, anything over 400 microns significantly increases the risk of these defects. Before you start thinking, "I’ll just apply all my films at less than 400 microns", keep in mind many systems need well over that thickness.

  3. They tend to be worse when applying on a heating cycle, i.e. during the late morning when the concrete is starting to warm. Ideally, you’re working in the afternoon when the temperature is falling and the conditions aren’t too hot (25oC is perfect).

  4. Some products suffer less than others and you may be lucky to have a local manufacturer that understands how to minimise these defects.

Benard cells are always lurking

In closing, if you didn’t know about Benard Cells and when they are most likely to occur, the chances are you’d simply blame the product for misbehaving. As I hinted at before, the Benard Cell is actually responsible for several other common film defects, e.g. colour separation in pigmented films, so it’s definitely a subject worth getting on top of.

Have you ever seen epoxy hail damage? What other defects have you seen that you’d like an answer to?

Take care and keep smiling,


Want to learn more about this and related topics? We suggest the following Epoxy School courses:

Bronze Card Course

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