I sculpted a custom base and as soon as i sprayed it with my black spray i use for minis it started melting away very fast i use a solvant based spray not the gw one you think it will be different ? is that one waterbased ?
I am not certain but I do not believe there will be any difference with the GW one. This depends on the Polystyrene type you have, I have had success in the past by spray painting from a longer distance, and only at shorter burst of like a second, then wait 2-3 seconds and another short burst. Guess what happen is that so little paint get on the polystyrene that it does not get the chemical reaction. Other than this, there is specific products that are meant to be coated on the Polystyrene to prevent this reaction. Or you can use water based paints, but not sure if you can get that on spray cans?
I have cut out Anwil of Doom years ago from polystyrene and sprayed it with GW chaos black. The surface was literally melted. I have heard, that u can cover polystyrene in PVA glue in order to avoid it demaged. I haven't tried this method though.
^ will work, pva the surface first and leave to dry. If you have an airbrush you can spray with acrylic paint or apply by brush. Beware of using water based sprays, if you apply a thick coat it will not adhere and peel off. Method I have used in the past is cover the surface with a thin polyfiller layer.