someone once asked: I have this dusty and rough concrete surface , can I fix it if apply over with an epoxy coating?

My response is “Apply at your own risk”. The only valid reason why someone should apply epoxy over rough concrete is to eliminate dust and protect the surface from absorbing water or other chemicals.

Below I explain three things that could go wrong:

1. If you think that the concrete substrate looks uneven, just wait till you coat over with epoxy. Epoxy is glossy and this will lead to all dents, cracks, holes and trowel marks of the concrete being revealed and magnified. The floor will quickly pick up dirt (since it will not be smooth) and very soon your coated surface will look very dirty

2. A common misconception with rough concrete is that by applying a high build epoxy coating (like a self leveling) on uneven concrete the floor will magically become smooth and level. THIS WILL NOT HAPPEN!  You run the risk of consuming a lot of product with absolutely nothing to show for. You cannot use a runny product that achieves thicknesses of 3-4 millimeters to cover differences in the slab that could easily exceed 1-2 cm. 

Also what seems like “I just need an extra bucket to fill that uneven area", can easily become 10 buckets (and it still won’t be even!!)

There are many different ways to level an uneven concrete slab but applying epoxy should not be one of them.

3. Epoxy coatings are only as strong as their substrates. In other words the substrate needs to have similar mechanical properties as the epoxy, otherwise it will break (see picture on the left). This is why many epoxy manufacturers recommend that the concrete substrate has a compression strength of 25 N/mm2. Would you ever apply epoxy on a soft turf? Probably not! Well applying on weak concrete is not much better.

Tip: whenever you find pieces of epoxy coating peeling, turn it over to check if there is any concrete stuck to it. In the picture below we can see a classic example of the top of the concrete surface breaking and getting stuck to the coating.

Concluding, I want to repeat what I said at the start of the article: only apply epoxy on uneven concrete if your sole purpose is to eliminate dust. Be prepared to need to re-coat the floor or perform patching jobs within a year or two. If you are a contractor I strongly advise against taking on jobs like these. Even if the client is on-board with the potential risks, the poor results could tarnish your reputation.