First show I ever worked on was what about 28 years ago was "Gidget goes Haiwian" We used scrap lumber,
chicken wire and lots of paper mache. Years later and under the rock in running the
smoke machine for .... forget the opera about a rat given it's English and I love it... or was it the opera "Hansel and Grettle"... I forget. It was about the same method but with
muslin covering the rock this time. No mice eating the scenery at least. Years later still in now designing and making a hovel for "Riders to the Sea" and back to no budget or time it was back to the
dutchman method and mice. Right after that it was Six Flags NY. I was working on at a scene shop and we were using like four
foot square by six
foot long blocks of foam we carved with chain saw and grinder chain saw blades for the rough cut and grinders, variable speed routers and belt sanders with sand paper wheels to hone and finish while wearing full
face masks and EPA suites because the foam got everywhere. This in addition to attaching blocks of foam to framing than shaping the foam. Large project, mostly all foam for their believe it was Ukon area.
In the mean time I also did spray paint and
torch burning of expanded polystyrine and extruded polystyrine (white dot foam verses the more dense colored stuff) for stuff like walls and rocks at times. Advantage of say doing a spray paint texture of the foam is if the spray paint is
flat or primer, the foam is ready to take latex paint afterwards. Otherwise one has to prime or Jesso it in making a surface that will take and not as easily flake the paint.
Dutchman and fabric coatings take paint much more easily on the other
hand. Takes extra work with foam to prime it for painting before the work can start on making it look like a rock. On the other
hand given a normal support structure for using
dutchman or fabric covering, it takes a
bit more work to make it look solid so the framing don't show
thru.
Never tried spray foam, remember my Uncle's truck from way back when that got spray foamed and such foam had shaping of its surface when not between layers the size of packing peanuts that would take a lot of sanding and work to get looking natural if that's the result of the surface texture. Expect spray foam these days even from a can is a lot more dense given at times large bubbles that would have to be filled. Filled in around my garage door with spray foam and the bubbles and expansion was also larger or more empty pocket bubbled than easily shaped.
Still though foam is fast to shape I believe but takes extra time to prime, typical rocks are faster to be ready to paint but take extra time to shape and
cover. Overall, spent many years in being stupid as a caution. Can't walk by white foam these days without a lung remembered rememberence of not wearing a mask or doing so in a well ventilated area. CPVC's in the air I believe it is ain't a good thing. At the shop proper stuff, off work for the night, just a small wall I was carving.