3 Questions for Using Clear Epoxy Resin

By Jack Josephsen

In the world of epoxy flooring, a good clear epoxy resin is essential as this type of product is often used for a wide range of common flooring tasks.

In the article below, we explore three frequently asked questions around the use of clear epoxy resin for three common applications - the polished concrete look, a pigmented floor coating, and finally, as a decorative clear topcoat.

Question 1: How do I get the polished concrete look with clear epoxy resin?

You’ve been asked to do a polished concrete or “warehouse” look for a client with your decorative epoxies - what do you need to know?

The traditional form of polished concrete involved 10-12 passes with a diamond grinder to hone the concrete into a smooth, glossy surface. While this type of flooring has been extremely popular in recent times, not every slab can be polished successfully and pricing is typically at the higher end. Because of this, a somewhat simpler and more cost-effective alternative can be done through the use of a clear epoxy resin to flatten the surface instead. This can be a very basic grind and seal floor, which results in the more rustic warehouse look, or, with the right product and thicker films, it can create a mirror-like, genuine polished concrete look.  

Either way, if you think the idea of slapping a clear epoxy resin over concrete is a walk in the park, don’t be so sure! Believe it or not, there are few traps that regularly catch installers out when taking on jobs like this.

Change your preparation goals

First and foremost, the polished concrete look and warehouse look will benefit from a slightly different approach to surface preparation. The natural inclination for many contractors is to whack a big, heavy grinder onto every slab and keep going until it’s completely flat. While there’s nothing wrong with that thinking as far as thorough preparation is concerned, ripping the tops of every high spot results in patchy, inconsistent aggregate exposure that rarely looks great in the final product. Instead, you want a grind that follows the contours of the slab and only removes the top layer across the entire area. An example of this type of thing can be seen on our Resin Flooring Tools page with a tool called Diamabrush, which uses diamonds on flexible brushes to do just that.

Clear epoxy resins aren’t dead basic!

Ok, so you have a freshly prepared slab with a relatively even “salt and pepper” appearance. From here many installers would simply mix up a clear epoxy sealer or decorative clear epoxy, pour it onto the floor to roughly spread out, and then start back rolling. The problem with this practice is you can often see a dark patch form under the thicker sections of the clear epoxy resin - what I call the “pour line”. You can never fix such a blemish if it shows through the final coat, so you need to prevent it by changing your approach when working with clear epoxies, i.e. working from a roller tray is generally a safer option on the first coat.

This type of staining effect can also cause headaches in other situations, especially when applying in warmer weather. In these instances, the hotter conditions lead to large variations in the viscosity of the clear epoxy resin, i.e. freshly mixed product is thin and soaks into the slab easier (looks darker), whereas older product that has begun to gel is much thicker and doesn't penetrate as much (looks lighter). The result of this viscosity difference is the appearance of noticeable bands across the floor corresponding to the rolling pattern used, and is particularly visible where an old mix meets a new one. Once again, there’s no quick fix and prevention through measures such as smaller mix volumes is the only way around it.

Another potential trap when working with clear epoxy resin is the formation of “holidays” or, in simple terms, missed spots. Unlike the staining that happens on the first coat, holidays are more common in latter coats when the finish is darker and the clear epoxy resin isn’t as easy to see. Unfortunately these defects always have a habit of standing out much more the next day and can only be rectified with extra coats and extra cost. I’ve found the use of low-level lighting to be an effective way of reducing the number of misses, however there’s no substitute for working carefully and using a slow, methodical approach to application.

Your clear epoxy resin must be capable

The final point I want to make on the polished concrete look and warehouse look is an important one: you have to make sure your clear epoxy resin is actually capable of delivering the finish you’re after. The main focus in this sense is how well it flows/levels and the gloss it can achieve. The warehouse look is pretty forgiving and many general purpose clear epoxies will be suitable for that type of finish, however, I know from experience there are only a handful of decorative clear epoxies that can make a good fist of a high-end polished concrete look. Regular readers of my posts will know I always come back to doing homework on products before committing to any projects, and it certainly isn’t any different here!

Question 2: Can I pigment a clear epoxy resin?

Have you ever seen a colour difference when rolling out a floor?

Let’s use a common example to illustrate the problem: you’re aiming for a simple light grey floor and go down the path of adding a pigment pack into a clear epoxy resin. You’ve already rolled out the first kit and it all looks good, however when you roll out the second you notice a clearly different shade of grey along the wet edge. Ouch! The client isn’t going to like that! 

It’s not always the epoxy pigment’s fault

If you don’t understand why this type of defect happens then you’re probably going to blame the pigment batch. That may well be a potential source, but what you’re seeing more than likely has something to do with physics instead. Let me explain.

Using the same example, we know that grey epoxy pigment is made up of mainly white pigment with a bit of black. In the epoxy industry, the majority of white pigments are based on titanium dioxide, which is a heavy pigment with a density of around 4 (compared to water at 1). There are different forms of black, but let’s say the black in this example is a carbon black, which has a specific gravity around 2.7. Anyway, if you put them both into a low-viscosity, clear epoxy resin then the heavier pigment will tend to settle at a different rate to the lighter pigment and, in a nutshell, that’s what causes colour separation.

What’s the answer to epoxy colour separation on clear epoxy resins?

So, if we know that pigments settle at different rates, how do we control the problem? The key point to pick up on here is the epoxy pigments can only settle quickly if the resin is thin enough, i.e. it has no “body” to suspend the pigments. With that in mind, to avoid colour separation you’re far better off using a tintable epoxy rollcoat, which means it’s designed to be used with pigments and will be specifically formulated to stop this settling from taking place.

I realise, of course, that sometimes you may not have a choice but to pigment a clear epoxy resin, such as a clear epoxy sealer. If so, the best thing you can do is give it some extra body by adding a bit of filler yourself. There are lots of different fillers to use and that’s a topic all on its own, however a couple of kilograms of the right filler in a 12 litre kit will make a big difference (Note: if you’re splitting kits then adding filler will change the volume ratio). There are also other ways of adding body to a clear epoxy resin by using specialist thickeners, but we might go into that later.

Other epoxy colour separation issues

To finish off, here are some other things you should know about epoxy colour separation:

1. Some colours will show colour separation more than others. The biggest danger is when combining two or more pigments together, e.g. black and white in light grey.
   
   
2. Some clear epoxy resins will show colour separation more than others as well. Additives used in the formulation can have a big impact, good or bad.
   
   
3. You also have to keep in mind that temperature will also affect the viscosity of the resin; in other words, colour separation may not have happened in winter, but that doesn’t mean you won’t see it in summer!

Question 3: What causes "hail damage" on a clear epoxy resin film?

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 clear epoxy resin films with lots of little dimples or dents and looking like a car that’s just driven through a nasty storm.

Tiny cells cause dimples on clear epoxy resins

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!

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 low-viscosity, clear epoxy 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 decorative systems need well over that thickness to ensure a flat, glossy finish.
   
   
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 (25^oC is perfect).
   
   
4. Some epoxy coatings 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 so it’s definitely a subject worth getting on top of.

Have you ever seen epoxy hail damage on a clear epoxy resin? 

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Want more information on clear epoxy resins?

You can find more flake epoxy flooring information on our site through the following links: 

Read our article on epoxy concrete sealers - What are epoxy concrete sealers?
Read our article on grind and seal flooring options - Do all grind and seal floors look the same?
See a polished concrete floor example - retail outlet
Watch our How To Video on using a clear epoxy resin to get the polished concrete look

Clear epoxy resin gallery

How you can take action...

• Products - Use quality clear epoxy resins that have been designed to form high-end films and overcome common imperfections:
  - Artepoxy Liquid Crystal - Clear Decorative Epoxy Resin
  - Artepoxy Liquid Marble - Metallic Epoxy Floor Coating

• Learning - Learn more about the effective use of clear epoxy resins for polished concrete and other systems with our online courses:
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