We used to have different extruded polystyrene in France: orange (a bit "sandy"), green (high density) and now I can only find yellow one which is very high density.
I didn't use much wall joint (in fact the thing you use before applying paint on walls to get a flat surface) on it as I didn't want it to be too heavy (I had to mask some cuts only and the joints betwwen polystyrene sheets). First I sand the polystyrene so it doesn't have a shiny surface (the paintjob is easier this way as long as you use brushes like me and not sprays or airbrushes).
Then I melt it a little bit with a heater (I don't know the english name but it is used to scrap paint with heat). It gets a "stone" texture with this step and it hardens the polystyrene. Then you sand it a little again and you can paint it.
All the rocks or tiles are scribed with a pen or a pencil (no knife at all except to sculpt large rocks).
But if you have a look at my previous Alkemy temple I painted straight on polystyrene after drawing stones and sanding it.