Building my first terrain table, need advice

Krylon H2O is a latex based spray paint that does not melt foam. I usually find the stuff at Hobby Lobby or other craft stores. I like to buy the cans that go on clearance (usually purples or other less used colors unfortunately) and use it to quickly seal foam projects and then use other sprays over it when it dries.

i sometimes paint larger style styrofoam with watered down PVA (wood glue) which can help smooth a little plus stops spray propellant from melting it, sand textured masonry paint is also good as it is lates based + the sand texturing hide larger polystyrene bubbling.

Hope this helps