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How To Make Foam Rocks: An Easy Guide

Whether you want to redecorate your lawn or design a new backyard, you often need fake rocks. Foam rocks can be a very useful and feasible formation to beautify your lawn. Expandable spray foams can be a good ingredient to make such rocks.

Now, start small to ensure the completed product is the size you planned because the material grows to at least twice its capacity. 

These spray foams are often used to fill up gaps around houses so using them to make fake rocks can be a bit tricky. Let’s have a rundown on how to make foam rocks.

Tick These Off Your Checklist First!

Before starting off to make the foam rocks, check off a few things you will need:

  • Cardboard or a plastic tarp
  • Rubber gloves
  • Putty knife
  • Rasp
  • Bread knife
  • Acrylic paints in similar rock colors
  • Disposable plate
  • Sponges or rags
  • Paintbrush

Process Using Expandable Spray Foam

Step 1: Ensure that the work area is secure and safe.

Cover the work area with a sheet of cardboard or a plastic tarp in a well-ventilated room, ideally outside. You will need ventilation with all those fumes from the sprays. If you’re going to make a rock out of cardboard, make sure it’s bigger than the rock you produce.

Step 2: Give the Can a Good Shake

Shake the expanding foam container for 30 seconds or as directed on the label. This may differ depending on the type of spray foam you are using. This also brings up and mixes all the residue in the spray bottle if your can has been sitting around for a while. 

Step 3: Safety First!

Remember, safety is very important. Often such spray foams can be harmful or toxic to your skin. The rubber gloves will protect your hands while wearing safety glasses can protect your eyes.    

Step 4: Attach the Nozzle

Remove the top from a can of spray foam insulation and spray the foam onto the waxed paper on the cardboard using the provided spray nozzle. To make a foam mound, move the nozzle around. Allow the foam to expand and cure as directed by the manufacturer.

Step 5: Make the Basic Rock Form

Spray the foam in a tiny spiral on the cardboard or top, similar to how you would a can-based whipped topping on a dessert. Continue spraying gently to form a rock-shaped mound, taking note of the expansion as it happens.

To allow the foam to expand, stop spraying long before it reaches the required rock size. As required, spray additional onto the structure once the expansion has stopped. Create rounded edges and angles to mimic river rock, or sharper edges and angles to imitate crushed granite.

Step 6: Creating a Shape for the Wet Foam

While the foam is still wet or sticky, shape it gently using gloved hands or a putty knife, generally during the first five minutes after spraying. Once the basic shape is in place, or if shaping proves too messy while the foam is wet, let the foam dry for eight hours or as advised.

Step 7: Shaping the Hardened Foam 

Using a rasp to remove tiny fragments of the material or a bread knife to slice bigger sections away, shape the foam once it has set. Small flaws can be smoothed over using the rasp. 

Step 8: Apply Paint to the Faux Rock

For a more authentic impression, paint your final faux rock with various tones of the same basic rock hue. Pour a little amount of tan and brown acrylic paint, or dark gray and white acrylic paint, onto a disposable plate, for example. Dip a sponge or cloth into a little amount of each color and dab it all over the imitation rock. For a more natural effect, use another cloth or paintbrush to mix the colors slightly on the completed rock.

Tip: If you are running out of spray foam, you can create a hollow rock form. Place a container upside down, such as a bucket or plant pot, then spray the foam on and around it to create the rock formations. Once the foam has cured, the original container or bucket can be kept in the rock or removed.

Extra Steps

  • Allow the foam to dry completely after removing the sanding dust with a wet cloth. Apply self-stick drywall tape to the formed foam rock.
  • In a 5-gallon bucket, mix quick-set concrete. Pour the concrete powder into the bucket, then add water and stir until the mixture resembles peanut butter.
  • With a trowel, apply a thin coating of quick-set concrete to the rock formations. Using the drywall tape as a guide, smooth the concrete over the rock until it is entirely covered. Allow the rock to cure before applying a final coat of concrete that is thicker. Allow at least 24 hours for the concrete to dry and cure. To aid in the curing process, moisten the concrete with water.
  • Use a paintbrush to apply concrete stain in hues that match the sort of rock you’re producing. River rock should be dark brown, tan, or other subdued hues. Granite rock gray with black specks is a good choice. Allow at least 24 hours for the stain to cure completely.

Tip: If you want to keep the rocks outside, use exterior latex paint instead of acrylic paint. This will protect the color of your rock from rain, sun, and other external factors.

  • Apply a transparent waterproof coating on the rocks to finish the job. Apply clear sealant spray to the whole surface of the concrete rocks. To produce a long-lasting waterproof covering for the rocks, use the clear spray as suggested by the manufacturer.

Using Spray Polyurethane Foam

  • You can make fake rocks with polyurethane foam as well. Chicken wire, wire cutters, scrap cardboard or paper, gloves, spray polyurethane foam, paint, sponge, and other materials may be required.
  • Using wire cutters, snip the chicken wire into the desired rock form. Because the application of foam will add bulk, make the chicken wire shape a little smaller than you want the completed rock to be. The rock will keep strength and body thanks to the chicken wire shape.
  • Using waste pieces of cardboard or paper, fill up the empty regions of the chicken wire shape. Filling the inside of the form allows you to use less foam while maintaining the rock’s shape.
  • Using a spray gun, apply a thin, uniform coating of foam to the whole structure.
  • Apply a second layer of foam while wearing gloves. Shape the foam with your gloved hands to obtain the desired rock appearance. When sculpting the imitation rock, make a copy of a genuine rock or a photo of real rock. While the foam is still wet, work fast.
  • Allow the foam to dry completely as directed by the manufacturer.

Final Verdict

Making foam rocks can often seem futile at times. The hard work and efforts only to get a few rocks can be tough. However, the outcome and aesthetics of your lawn and backyard make it worth it. Let us know how you benefited from our article on how to make foam rocks and don’t forget to show us the results.


Question 1: How do you make fake slate rocks?

Answer: Combine 3 parts sand and 1 part portland cement in a mixing bowl. Depending on the size of the rock you’re making and the amount of mortar you’re mixing, combine all of the materials in a wheelbarrow or concrete mixer. To make a more porous fake rock, decrease the sand and add 1 part peat moss.

Question 2: What does polystyrene look like?

Answer: Polystyrene comes in two forms: solid and foamed. Polystyrene is transparent, rigid, and brittle plastic.

Question 3: How do you make fake rocks out of boxes?

Answer: Make the frame of your rock out of a cardboard box or polystyrene foam. Then, wrap the frame with paper mache to give it a rock-like texture. Allow the paper mache to dry before spray painting it. Your faux rock is now ready to use!

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