Well I don't know any brands that don't eat foam/ polystyrene. However if you coat your bit of scenery with watered down PVA glue, then it normally protects the polystyrene from the spray. Its the propellant in the cans that affect the polystyrene, so read the sides and look for some to try. I myself have found that if you do a lot of scenery, then purchasing a small modellers spraygun and watering down the paints is a great idea.
All standard spray paints woill contain solvents, the best way to get around the problem with melting polystyrene is by getting your hands on a small hobby compressor and spray painting equipment. Then just spary paint with the standard GW paint or any other water solution paint.
All spraypaints will eat away at polystyrene, because they all include solvents in the mixture. Solvents like cerosene, acetone and so on.
Do as the others have tip'ed here and give the object you're gonna spray a good cout of glue or paint or what ever you like and then spray it.
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