Rocks are useful for all sorts of things, such as landscaping and aquarium decorations. However, rocks are heavy, cumbersome and not always in the shape or colour that works best for your project. Foam rocks are great props for plays, toys for children or a place for a secret key. Make foam rocks from styrofoam, also known as polystyrene, for a lightweight alternative to regular rocks.
Glue sheets of polystyrene together to build up the size that you want the rock. Foam is available for purchase at home improvement stores, or use the foam packing material from a box. Carve out the shape that you want the rock to be with the craft knife.
Mix the grout a bit thinner than the manufacturer recommends and apply a thin layer to your rock. Let this dry for 24 hours. Look at the rock after it dries and make sure that it is the shape and size you wanted. To make it a little bigger, you can mix more grout that is just a bit thicker than recommended and make a thick layer. Allow it to dry for at least 24 hours.
Apply a layer of marble spray paint to the top of the rock and let the paint dry. Then, flip the rock over and paint the bottom. Allow the paint to dry completely and add another layer.
Apply a water-based sealant after the paint has completely dried and allow the sealant to dry before using the rock.
Choose a matt finish for the sealant, or the rock will look shiny.