Making artificial rocks is an important project for plays, movies and landscaping. Artificial rocks are lighter than real rocks, and usually less expensive. One easy way to make realistic looking rocks is to create the rocks from styrofoam and polyester resin. After making the rock base you can then paint the rocks as desired to make them look realistic.
Cut the styrofoam into the basic shape that you want for the rocks using the utility knife. Use duct tape to hold the pieces together if you want to fit more than one piece together.
Cut fibreglass sheets into 4-by-4-inch pieces. Use smaller pieces if you want to build rocks that are smaller than 4 inches. Wear thick work gloves to protect your hands.
Mix up a small batch of polyester resin. Only mix up enough for one coat at a time. Add enough catalyst to make the mixture thin enough to spread but thick enough to not drip.
Dip the fibreglass sheets into the resin mixture. Press the sheets over the styrofoam blocks. Overlap the sheets when possible, and press down the edges to create rounded corners. Allow the first coat to dry overnight.
Fill in the tops of the fibreglass sheets with another coating of resin applied with a paintbrush. Allow this coat to dry overnight as well. Apply a second coat of fibreglass sheeting and one final resin coat. Allow the rocks to dry for 24 hours.
Sand the surface of the rocks with rough grit sandpaper to create a rough surface similar to a real rock. Wipe the rocks clean with a damp cloth.