Why Do Bubbles Form In The Epoxy Screed?

One of our suppliers advised us that at a recent trade show in Dubai he was surprised to hear from a lot of different visitors, from different countries, complaining that epoxy screeds form too many bubbles on the surface. What they were talking about was that once the screed cured it had bubbles which resulted in an unsightly looking surface littered with little holes that are very prone to picking up dirt resulting in a very unsatisfactory floor impression.


It was very surprising as to the apparent lack of knowledge regarding the cause of these bubbles. Any experienced epoxy applicator should be aware of what causes bubbles and what we can do to avoid them. Bubbles in your finished screed can be attributed to a number of reasons:

  1. Problems with the substrate. These are usually caused when the substrate is absorbent which leads to air being released
  2. Poor and/or inexperienced application techniques. Most problems associated with bubbles are related to something going wrong during the process of the installation.
  3. Problems with the product applied. This is a growing problem whereby many low-cost companies are emerging more and more with questionable product quality.

In this article, we deal with problem No.1 (the substrate).

Now whilst we all know this is a fundamental rule, it needs to be said that when applying epoxy screeds in any conditions on a porous substrate it is vitally important that the preparation has been carried out to ensure that it has been properly primed and sealed.

Quite simply, bubbles emerge when the substrate is of cheap and porous quality. You will find that this is very often the case with cement or concrete substrates in which the composition of the mortar is not according to standard application procedures such as using an excessive amount of sand or water.

The substrate will begin to absorb part of the screed which results in air being released inside the screed. This is where the holes and huge ugly craters begin. But be warned that it is very often these holes don’t emerge straight away and the application looks great … until an hour or two later.

The only solution to overcoming poor substrates is to properly seal the floor with several layers of quality epoxy primer until the affected surface has achieved a smooth glassy texture. Provided the budget allows, it is always a good idea to apply additional layers on top of the primed surface regardless of whether it is just a simple coating or a screed, before you apply your final coating.  It is well worth noting from the outset that a well-sealed, prepared and primed surface goes a long way to ensuring the best possible results. But ideally, what is of primary importance is to have a satisfactory substrate to begin with.

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