We mixed it very thin and then painted it on with brushes. I scrubbed the outside of the styrofoam bricks with a wire brush first in the hope that it would allow the concrete to bond better. We'll see. The blue foam seemed to absorb the cement mixture which will probably allow for a stronger bond. I'm slightly worried about cracking. Should be ok.
We're going to wait two days to let it dry completely and then put on the second coat.