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THEY'RE EASY (in HO scale)

Written by Rick Smith (a member of Cape and Islands Model Railroad Association)



Mountains, rocky hills, rugged seashores, or even that little mound in your backyard… they're all made of rock. This Earth is made of rock. Even sandy islands like Martha's Vineyard have some exposed rock. Most of us would like to see our model railroads run through at least some rocky terrain.

 How can we create rocky terrain for our layouts? Here are some of the techniques that we are using at CIMRA (Cape and Islands Model Railroad Association) to build the mountain range. The techniques are adapted from the Dave Frary's book "How To Build Realistic Model Railroad Scenery", manufacturers instructions, various articles in Model Railroader and Model Railroad Craftsman, and our own trials and errors (lots of those!).

What makes a rock look like a rock? Have you ever picked up a piece of rock that just crumbled in your hand? You were surprised! It looked like a hard rock, but it didn't feel hard. A rock doesn't have to be hard to look like a rock. In our model worlds there are two things that make up the look "ROCK".

That's it. It doesn't matter what the ROCK is made of. It could be plaster, foam, paper mache, or ceiling tiles. But the texture and color must be right to convey the impression "ROCK". 

We wanted lots of rock done quickly. After all we had a lot of mountains to build. They had to be durable. A club layout sees a lot of abuse. So we decided to make Hydrocal plaster rocks from latex rock molds. The Hydrocal is harder than regular plaster; and it sets up faster. The latex molds would produce a lot of texture in a short time. 

To get the shapes I envisioned I decided to make rocks molds for the club. The hunt for a master ROCK was on! I scoured the beaches, road cuts, and backyards for chunks of rock that looked like miniature rock faces. Some rocks will erode in a pattern that resembles rock cliffs or blasted rock. I found a large sandstone boulder that had the texture I was looking for. But this was in February! It was too cold for making a latex mold outside. I wanted to work in a warm basement. So I returned with hammer and chisel. 

The problem with trying to cut off a chunk of rock from a boulder is that the rock will split along the seams and ridges. But these features were what I was trying to capture. After a few hammer blows to the fingers I succeeded in ruining several nice areas of boulder. Kicking the ground in frustration I turned over a loose rock. EUREKA! There was my first master rock waiting to be carried home. Which leads us to the first task of making "ROCKS": 


If you can slop some paint on a wall, you have all the skill it takes to make a latex rock mold. Even if you don't have that skill, or don’t have the time or desire there are many commercial rock molds available. 

You need: 

When you buy the mold compound shake the can. If it doesn’t sound liquid it may have hardened in the can. 

    1. Now, wash the rock. Yes, I'm serious! Use dish detergent for the soap. A scrub brush helps remove anything that isn't rockish (lichen, dirt, twigs, bugs).

    2. Leave the rock slightly wet for this next step. The water helps the mold compound to sink in to the crevices.

    3. Brush on a coat of the mold compound covering the area you want to copy. Don’t let it puddle in the cavities, brush it out.

     4. Let this coat dry. Overnight is good if you have time. If not place a 100w light bulb in a reflector near enough to warm the rock. This will speed drying to about 3 hours. If you're really in a rush, you could warm it in an oven to dry it.

    5. Add 3 more coats.

    6.When you paint on the 5th coat you will add gauze or cheesecloth to strengthen the mold. Paint on the mold compound. Then place the gauze on the wet surface. Use the paintbrush to stipple (poke) the gauze into the crevices and to add a little more compound. Let dry.

    7. Paint a 6th layer over the gauze to fill in the weave.

    8. Now, let it dry overnight.

    9. When fully dry you can peel off the mold. Start in one corner and roll the edge up with your thumb. Carefully pull this edge until the mold separates from the rock. you can trim the edges with scissors.


Your new rock mold is ready to use!


  1. NOW WHAT DO I DO? - CASTING STONES (or rocks)


Since we don’t live in glass houses its OK to cast stones.


Casting rocks for model railroad scenery means:

It almost takes longer to tell you about it than it does to do it!

 You need:

Add a couple of drops to dish detergent to a spray bottle and fill it with water. Spray this soapy water on the inside of the mold. The water helps the plaster flow into the little crevices and other details in the mold. The soap prevents the plaster from sticking to the mold.

1. Place the rock mold on a flat surface with the inside facing up. You can prop up the edges to prevent the plaster from running out.

2. Mix a soupy batch of plaster

-Put a cup of water in a plastic mixing container. I use empty yogurt containers. They can be flexed when the plaster hardens and the plaster breaks and falls out.

-Add Hydrocal plaster to the water while stirring. The mixture should be thin enough to pour easily, but not too soupy. About 2 cups of plaster should do it.

3. Pour the plaster in the mold. Use a spoon or stir stick to push the plaster into the corners. Let the mold sit for a couple of minutes. When the plaster starts to thicken, but is still soft it will be ready to apply to your scenery.

4. Wet the area of the scenery where the casting will be placed with the soapy water. This prevents the scenery from absorbing the water from the plaster before it sets.

5. Press the plaster side of the mold against the scenery. Be sure to press down all of the edges of the casting. They will be sticking out when you remove the mold. Now hold it there for a couple of minutes. You can test the setting of the plaster by lifting the edge of the mold. If the plaster stays on the scenery you can stop holding the mold. If the plaster stays in the mold you need to hold a few minutes longer.

6. When the mold feels warm you are ready to peel it off. Lift up one edge. Peel back the mold, turning it inside out. If it gets stuck under the edge of the plaster, stretch it. The mold will usually pop out. If not, you may need to break away a little of the plaster.

Now, step back and admire your ROCK!

Here are some hints and solutions to problems you might run into.

What went wrong!?!?!? - Dealing with problems.


Q. The plaster ran out of the mold when I put it on the scenery.

A. Either the plaster was not left in the mold long enough to set or it was mixed too thin.

Q. The casting was cracked when I removed the mold.

A. The plaster was left in the mold too long and had hardened before it was placed on the scenery. Actually, if it's not too bad a crack this could look good. Take advantage of it or patch it.

Q. The edge of the casting sticks out in the air.

A. This happens if you don’t press down all the edges and hold them while setting. It might also happen if the plaster really set up hard before putting onto the scenery.


The next time you are on Cape Cod, stop in and visit our club layout. We are located in Hyannis, behind the Cape Cod Mall, at 167 Corporation Road. We occupy the top floor of Centerline Hobbies. If you would like to see these techniques in action visit the club on a Wednesday night between 7:30 PM and 9:00 PM. Wear your old clothes and we'll sling some plaster together!

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