What kind of foam for diorama rocks/cliffs?

Building and painting are two separate things, right...but for building, I've been a fan of the solid pink foam stuff you get at Home Depot and the likes. Or even left over blocks of standard issue white styrofoam. I think it's easier to carve and shape than the spray foam. BUT, I think the spray foam lends itself to some interesting shapes that maybe don't even require carving.

For painting, I've just used good old acrylic paint and done my best. Nothing fancy there.

I use mostly pink foam now, but some of my previous scenery projects used styrofoam, 

like the big chunks that are in boxes for packing

if you wash it w/ soap and water it gets the mold release off of the surfaces, and glues better

first coat is 50/50  elmers school glue/paint, of the "dominate" color

the next coat I use 50/50 wood glue/paint (helps it resist moisture)

and while it's wet, I dust it with colored sand, and fine ground foam scenery stuff

www.ebay.com/itm/395256097596?var=0&mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338590836&toolid=10044&customid=34c8b9355c001b84aaefdf452d82272b

matte hairspray can be used as a final coat, and whenever the trees and shrubs look dusty

dr

Pink/blue the best, very hard to get.  Use a foam cutter, and get as dense a white foam as you can, look for 6 or 8 inch thick at construction sites (ask first, or wear khaki,  a short sleeve light blue button shirt, and a US Presidential seal cap, carry a clipboard, and....nevermind, just ask nicely for scraps)

youtu.be/RKiiR1BPh2s?si=X8_Q-rj2xUNCR0ff 

I found this video to be very illuminating on the topic. 100% with the comments above

(They actually build stuff; I'm just a fan)

Follow-up...with a post I made showing how I did scenery for my slot car track several years ago. It was literally trash piled up and then covered with the glue+paper towl combo, then painted. It worked very well and looks pretty good IMO. I didn't have to purchase anything extra, just used what I had.

www.redlinederby.com/topic/making-mountains-building-up-the-scenery/3927

I've seen a few tracks, skip the foam and used RAM board, or rolls of brown paper, if you crumple it before applying it, it gives a very realistic Brown Rock appearance. It would probably be easily and spray painted. 

I've used a mix of insulation foam panels and spray foam. The spray foam is great if you're trying to make some odd shapes but if you cut into it, shows the air holes in the foam. The insulation foam is great and easy to work with but is trickier to get large build up areas as you have to layer it all. I stay away from normal polystyrene usually, mostly due to the level of mess with it and trying to clean it up is worse than the other two options.

Model railroader here. When I did a couple of layouts, I used foam boards cut and stacked whatever way I needed it, then glued all the pieces together. You can create tunnels, mountains, roadbed, whatever you need with little effort. I've also used yardsticks as bridge roadway supports, vinyl tape for roads, and coffee stir sticks for fences, sidewall, docks and deck material.

Just did a week build 
painted paper to a decent start to a scene
as spy dude said, use any materials, industrial paper mache, gravel, ground leaves,
the best grey fine road gravel/ballast, go out to the road with a broom and dustpan (ignore the weird looks from the neighbors)
use a frypan screen to get the fine stuff, and grade up with bigger screen
and real dirt makes great scale dirt (who'd have thunk that...lol)

any questions ask away

dr

www.youtube.com/watch?v=czfrxC-qHnQ

I prefer the insulation pink foam board. Here is my announcer's booth using it with acrylic paint. Thunder Mesa Studio is a huge inspiration along with Black Magic Craft and Wylock's Armory, this guy is a wizard with just cardboard. I am also building my city and other buildings with coffee sticks etc. Great Ideas.