Give the fireplace a base coat with white paint. No matter what material you used to build the fireplace, you will have its own color and texture with its own inconsistencies. For the best imitation effects, you want to work with a neutral base.
Sketch the contours of the stone shapes. Describe either irregular natural stone shapes with roughly round and oval drawings in different sizes and positions, or draw ribbed brick patterns if you want a fireplace of carved stone stones. the latter, achieves consistency by cutting out a rectangular cardboard pattern to trace for each brick.
First paint the wall. gray color and allow to dry. Create the impression of pores in the mortar by spreading the painting over this first coat in a slightly darker shade with the same color, apply paint to an old toothbrush and run your finger through the brush to remove stains of paint. Let it dry too.
Add the stone detail. Fill the stone contours with a solid coating of light earth tones (beige, white, or gray), then add a partial, splotchy second coat of a different color with a sponge. You can also use a granite effect spray by covering the ‘mortar’ sections with masking tape, spraying and removing the tape.
Create shadows around the bricks. Paint dark contours around the sides and bottom of the stones. Make the bottom lines a little thicker. This will give the illusion of dimension.
Like most large props and sets, the best fake fireplace for a stage production is one that is lightweight, yet real. Achieving this effect requires creating a realistic stone structure in color, one with compelling colors and realistic depth and dimension.