How to Make a Fake Rock Wall

Hang sturdy paper on wall using a staple gun. Staple every 4-6" along the edges to ensure that the paper will hold the weight of the foam and portland. Overlap paper about 6" and seal off edges with packaging tape. Be sure paper is cut to about 1" larger than the area you want to cover. It will be trimmed at the end.

Put on gloves and lay down a tarp. This foam is very difficult to remove from skin and other surfaces (tile, carpet, walls, etc). If you don't want foam on it, cover it up.

Apply foam to wall, leaving about 1" around the edges to allow the foam to expand (again, we will trim this later). Shake the can thoroughly and hold it upside down when you spray the foam (as per instructions on can). Spray foam directly into gloved hand and smear a thin layer (1/4" or so) onto paper. Don't worry about spreading the foam into an even layer. Uneven layers will add texture and shape to your wall. As round bubbles appear, smooth over them with your gloved hand to shape the texture. Add any details now (like crags, shelves, etc) with the foam. Change gloves as needed (you may find that you need to change gloves after each can). Foam will expand for about 30 minutes. Cover entire area and let dry for approximately 2-4 hours, depending on how thick you applied the foam.

Once foam is dry, mix the portland in a five-gallon bucket with enough water to create a paint-like texture. Using a large brush, "paint" the portland mixture onto the wall. It helps if you only mix a little at a time (make about 2 gallons, use it, then make more). Let portland dry on the wall overnight (or approximately 8 hours). Portland will dry a light grey color, but you can add cement dyes or stains in if you wish. These can be picked up at your local hardware store.

Once Portland mixture is dry, use textured spray paint to create shadows and details on your wall. Pick colors that are natural and would look good with your other decorations (e.g. a spray paint with a thick, large-grained texture).

Trim paper around edges (remember the 1" we left?) with an exacto-knife or razor blade.