Finishing foam rocks

[Carlo]

I would like to try making a rock wall using cubes of blue polystyrene foam, as seen below (from Facebook).
I like the general look and variations shown. Does anyone know of a tutorial or explanation of finishing
techniques to round edges, smooth, score, and texture the rocks for a more realistic look?
Plaster, thick paint, gesso? All help and links appreciated...
Carlo

[5thwheel]

Have you tried tumbling them?

[detail_stymied]

maybe some help:

https://www.nebrownstone.com/blog/working-with-new-england-brownstone-castings/

http://www.terragenesis.com/mybb/showthread.php?tid=353&page=14&highlight=foam+carving

http://www.finescalerr.com/smf/index.php?topic=1806.0