bojan, I'm not sure what sort of finish polyurethane gives you but it might be worth while looking at a arcrylic render finish.
Taulbmans armawall roll on render can be applied directly to styrofoam, no mesh, and gives a hard finish. It comes pre mixed in 15L containers and before application looks like really thick paint with .5-1mm stones through it. I applied it to mine with a trowel and once set, I gave it an acrylic texture top coat. Mine was hit by cricket ball a while back, leaving a small impact crater which I just slapped some top coat texture over to seal it. It's pretty tough stuff.