- Natural rocks are shaped by time and weather, so any rocks you make should mimic that. One of the best molds for artificial rocks is made from chicken wire, which is also very easy to manipulate and form as you desire. You may find it does some of the shaping naturally. If you're making multiple rocks of about the same size, alter one mold slightly between uses.
For a hollow rock made of concrete, start with a mold for the inside before making the outer one; this gives you the right sense of scale and lets you plan the thickness of the rock. If you're putting the rock over something, make sure the mold is at least a few inches larger than the object on all sides. Test the inner mold over the object to be sure it will fit.
If this is your first time making artificial rocks, start small. Larger and more complicated rocks are easier to do incorrectly. Making smaller rocks will give you some experience before you move up to larger ones so you can apply any knowledge you gain to more difficult projects. - Make artificial rocks out of concrete or fiberglass. One is considerably heavier and can withstand more from nature, while the other is far lighter and more portable. Which you choose depends on what you intend to use the rock for and your comfort with either material.
For concrete rocks, be cautious about the amount of water. A really hot day will call for more water than a colder one, and damp sand means you need less water than if the sand is dry. Cover the chicken wire mold with burlap sacks before putting the concrete mixture into the mold with a wet trowel, spreading it in layers until you've filled the mold. As the rock dries, make sure it's not drying too fast--if it's cracking, spray it with a mist from your hose.
If you're using fiberglass, make a papier mache negative of the chicken wire mold. Most hardware stores have fiberglass kits that you can then cover the papier mache with. Some artists dip burlap into the fiberglass and drape it over their rocks to give it a rocky texture. - The best way to give an artificial rock a natural look is watering down some paint and using a sprayer. Put down a darker base layer of paint before doing any other work. If you want to add some kind of striping--known as striation--in the rock, use a piece of fabric as a stencil.
For concrete rocks, sealant will keep the paint fresh longer. You may need to periodically refresh the sealant, but some owners choose to let the rock weather to add to its natural look. For fiberglass rocks, consult the fiberglass kit's packaging to see what kinds of sealant, if any, can be used.
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