When applying epoxy screeds on a porous substrate it’s very important to ensure that the substrate has been properly primed and sealed. Bubbles may emerge if the substrate is of cheap and porous quality. The substrate ends up absorbing the screed. This leads to air being released inside the screed which causes holes and huge ugly craters on the coating surface. Often these holes do not emerge straight away but an hour or two after application. Even if you try to treat the problem with a spiked roller, the bubbles will keep on emerging.
The solution to poor substrates is to properly seal the floor with several layers of epoxy primer until the surface has achieved a glassy texture. Cost-permitting it is always advisable to apply additional layers on top of the primed surface (whether it’s a simple coating or a screed) before you apply the final coating. A well-sealed and primed surface goes a long way to assuring great results.