I’ve charged a show with foam rocks that had to withstand weight. Actors were able to stand, sit, jump, and walk on these rocks, which also had to able to roll on stage. Extruded polystyrene was applied to wood with green glue, and then carved.
We coated them in a scenic dope made of: plaster of paris, white glue, relatively fine sawdust, jaxsan, joint compound, a bit of water, and Rosco Off-Broadway paint. Essentially an extremely strong binder, some filler, some vehicle, and a bit of pigment to tint. The mixture was applied between layers of cheesecloth and took about 20 hours to dry (overnight). Worked really well! I can see how dry clay powder could also help in this situation.