Faux arch stones are applied to a surface or wall to create the impression of a weight-supporting stone archway. Stones can be made from lightweight, easy-to-cut and carve styrofoam sheets, which can be pasted to the wall and painted to render a realistic surface. Styrofoam is fast, easy to work with and relatively inexpensive, making this a good project for beginners, since each stone can be cut multiple times to get it right. Start with a picture of an arch you like for reference.
Hold a piece of cardboard against one face of the arch to which you intend to attach stone and trace the arch onto it for a template. Cut the template out with a box knife and place it on your sheet of 2-inch-thick styrofoam.
Trace the template onto the styrofoam. Use your reference picture to draw stones around the arch, onto the styrofoam, as they appear in the image, using a permanent marker. Work to create a balanced look, so that stones on each side of the arch are similar in shape, size and placement.
Cut the stones out with a jigsaw or band saw along the outlines. Use a Surform grater plane to rough up the edges, rounding them up into the face to create a smoother, more stone-like edge. Use a box knife to cut into the face of the stones, creating pits and making the surface slightly uneven. Sand each stone smooth with a random orbit sander and 150-grit sandpaper in a space with plenty of ventilation.
Choose three colours from your arch image. Select the main colour of the stone, a medium tone, one should be the darkest colour in the stone and one should be the lightest. Order enough to cover the stones with each colour. Mix them with standard white school glue, three parts paint to one part glue.
Brush a thick coat of the medium base colour over the stones with a disposable paintbrush. Cover all edges and apply the paint thick enough to fill in the bottom of every cut and divot. Allow this coat to dry thoroughly. Rinse the brush immediately in hot water and soap.
Pour small amounts of all three paint colours into open containers, such as paint trays. Use an art brush to paint the darker colour into all indentations in your stones. Add highlights around the rim of each area with the lightest colour, using the same brush.
Dip the larger paintbrush you used for the base coat into the medium, then the dark and then the light. Apply this over the surface of the stones working with an X motion to blend, or scumble the colours together. Allow this to blend lightly into the shadows and highlights you painted.
Brush all three colours onto a plastic "wool" scrubber with a handle. Daub this across the stone, while it is still wet, to add a pebbled texture to the surface, further mottling the paint. Allow the paint to dry overnight.
Install the stones with construction adhesive, or drive three inch drywall screws through them, running the heads just past the surface, if you want to remove them later. Fill the screw holes with painter's caulk and paint over them.