I need to do part of a job that will be a 4" thick piece with one "rough" side that will face out. Not quite a faux rock face but closer to a rough bluestone face. I was thinking of using a piece of 1" foam on that face of the mold and roughing it up then pouring on it. Maybe using some mold release on that face as well. Anyone have any ideas? By the way it is an SCC mix
slice 4" strips of clay, about 1/4" and put it against the mold wall. take a few real rocks and push them into the clay at varying degrees. Cast quick. when the clay dries it will shrink and pull away from the mold wall, also cracking and shortening in length
I've done the beat-up Styrofoam trick, and it works fairly well, very realistic. The challenge is to clean out all the foam bits. I would suggest coating the foam face with urethane and then waxing the crap out of the urethane before casting against it. Then brush the cast face with a 120 g silicon carbide brush.
I use clay also, but press the clay into a rubber slate,blue stone or granite seamless stamp mat (one you would use for stamping an overlay) and place it against the edge you want the texture.