White Styrofoam/polystyrene/eps Foam Block 12 X - Etsy
Foam Coat
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FoamCoat
UreCoat™ - The Compleat Sculptor
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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.