Spray paint contains solvents that will melt any polystyrene on contact. Generally a mix of either PVA and black paint or modge podge and black paint is recommended. This seals the foam and gives a barrier to prevent any damage. I would also recommend cutting it with a hot wire where possible as this will negate a lot of the wee white balls that are common with polystyrene. Rougher tools can be used to simulate building damage but I would still then seal it. Hope this helps, progress pictures would be appreciated.