White Styrofoam/polystyrene/eps Foam Block 12 X - Etsy
UreCoat™ - The Compleat Sculptor
FoamCoat
HARD COATING STYROFOAM! Cheap Way To Make Foam Props Strong (Big Halloween Pumpkins) - YouTube
FoamCoat
Terrain idea: Tried to smooth styrofoam, turns out enamel spray and acrylic clear coat have some interesting cratering effects instead. Could be useful. That said, like any spray painting, do it outside.