For that one I coated the polystyrene in 2 layers of grey mortar (most people use grout but I didn't think there was much difference), I also mixed with some pva in it to help it adhere better, then once all dry I coated it with matte mod podge to seal it fully.
It's solid as a rock, think it would quite happily survive a drop on the floor, not that I'd like to test .
Decided against paints for my first try, will probably paint next time.
Also mixing in coloured sands doesn't work out as well as you'd think. Tried red sand and came out looking like, well, something from a rear end .
Guess it's a bit of trial and error with that lol .