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.
All Purpose Foam Coat - The Compleat Sculptor
FoamCoat™ | Rosco
White Styrofoam/polystyrene/eps Foam Block 12 X - Etsy
EPSILON™ Series, EPS Foam Coating Epoxy | Smooth-On, Inc.
Gel Coat FRP Smooth Finish - China FRP Smooth Finish, Smooth FRP Finish | Made-in-China.com