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.
Foam Coat - 3lb – brickintheyard
ROSCO Foamcoat protective coating
All Purpose Foam Coat | Hot...
White Styrofoam/polystyrene/eps Foam Block 12 X - Etsy