Master Mold Making

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by David Barron

There comes a time when everyone who builds a model railroad needs a special tunnel portal, stone viaduct, cut stone bridge support, or pillar to support a structure, and the darn thing just isn't available in the commercial market -- at any price.  Most railroaders will needlessly give up the special idea and settle for something secondary, like cutting an existing tunnel portal, usually with weird results, or modifying the "perfect" scene to eliminate the problem.  The answer to this situation is to sit down and design, or copy from pictures, that perfect idea you had!  It's a lot simpler than most people have been led to believe; if I can do it, so can you.  Your tools, can be found around the house and with a little imagination, and some plaster, you are on your way.  Now let's get started.


The minimum tools that you need for this project are listed below.  These same tools are used when you put scenery on your layout, so if you don't have them now, you will sooner or later.

  1. A stainless steel straight edge, like a 6" metal ruler.

  2. An old toothbrush, preferably one you are finished with in the bathroom.

  3. A couple of X-Acto knifes with #11 blades, old dull ones will do.  If you are like most model railroaders, you're a packrat and have saved up a bunch of them.

  4. Some sort of a pick, like a spring steel wire poked into the end of a wooden dowel with the exposed end sharpened into a point.  This is to get those stubborn plaster pieces out.

  5. A comfortable chair, located in a place where itty-bitty plaster chunks can fly around without the spouse raising an eyebrow at your mess.

  6. Safety glasses to protect your eyes from those itty-bitty plaster chunks flying around.  You have no idea how much plaster stings in the eye until one of those chunks lands there.

  7. A bunch of large straight pins for holding the mold together while we are pouring plaster into it.

  8. Some smooth (beaded type) sided Styrofoam, the best kind has the metal foil on one side.

  9. A bottle of Mountains in Minutes green latex mold making goop, and some gauze and sticks to help stiffen up the mold.

  10. Some strips of aluminum sheet metal the same width as the thickness of your Styrofoam.

  11. Your favorite plaster.

Getting Started

Obviously, you have some special situation or you wouldn't even be considering this project.  Let's say that you need a special tunnel portal that you want to make, that will duplicate one you fell in love with from some where else.  First of all, study the picture of it and look for the hidden details like relief, brick or stone surfaces, wood, keystones, how the material was bonded together.  Look at its height, width, and approximate thickness.  Once you have developed a mental picture and a plan, make a sketch of it approximately the same size that you want it to be.  Make all your improvements on the paper now because once the plaster is poured you'll be in duck soup -- well maybe wet plaster.

Dam the Thing

Since you have decided on what the exterior size will be you can take the Styrofoam and start making the dams so the plaster doesn't end up all over the floor.  Cut a piece of Styrofoam about a foot square to be the base.  If you're working in a larger scale, you may have to cut out a larger base.  Next, cut some strips of Styrofoam about an inch wide but the thickness of your portal or project.  You can now form a square the same size as the proposed portal exterior size.  Use the straight pins to poke through the Styrofoam strips to hold them in place.  How do we get the arch?  The aluminum sheet metal is bent to the arch design that we want, and the legs are trimmed to the proper height and secured with a straight pin, on both sides of the sheet metal, inside the square with the leg ends firmly against the edge. A good source of sheet metal is a soft drink can, watch the edges as they are sharp after you cut them with a scissors

Pouring the Plaster, etc.

Mix your favorite plaster, with or without coloring, and pour it into the cavity you have just produced, up to the top of the mold.  To drive out the bubbles, tap the edges of the mold base for a minute or two.  After the plaster starts to firm up, not harden but just firm, where it will not run off the base, take the pins out and carefully remove the dams.  Carefully remove the arch as well.  Your rough formed master should hold its shape now and soon will be ready for carving.


Carefully draw your straight edge over the plaster face, several times, shaving the surface so it is perfectly smooth and flat.  Put on safety glasses if you have not already done so.  Take your drawing and place it on the surface.  Take a pin and poke small guide lines along the drawing to transfer the image to the plaster surface.  Take a knife or straight edge and carve out the relief.  Use the pick you made to get out any stubborn chunks.  To cut courses of brick use the straight edge and draw the #11 blade along the length of the brick pattern.  I like to use the backside of the blade to do this, not the sharp edge.  After you're happy with these lines, take the knife and cut individual bricks or cut stone by alternately cutting vertical lines connecting the parallel lines.  If you are doing brick, make sure the individual brick lines follow up the face by using your straight edge again.  Cut stone is not as critical.  You can freehand field stone by just making several non-repeating patterns.  Now comes the best step of all.  Reach over and get that old toothbrush.  Use the toothbrush to clean out the lines by firmly brushing both directions on brick and all directions on random stone.  The longer you leave the plaster set the sharper the edges will be.  This will not only clean the lines you just cut, but round the edges a little.  On brick you will want to brush one way, but not too hard as you want the edges to stay nice and sharp.  You might want to even delay this cleaning until the plaster firms up more.  On random fieldstone, I found that firmly brushing in all directions make the random stone more round and accents the space between the stones.  Don't forget to give some of the individual stones some relief as well.  If you're doing a non-concrete arch, you will need to put in a course of stone or brick with a Keystone at the top.  This locks the unit together in real life and is a necessary detail.  You can freehand cut this course of brick and the keystone.

When you are happy with the looks of this project, put it aside to fully harden and dry.  This will be your master, so treat it with tender loving care unless you want to make another one.


Now this is a touchy subject in most families, so I'll keep it brief!  OK, so my jokes are a little lame!

I use Mountains in Minutes green latex mold compound and gauze.  After the master is thoroughly dry, I start by force brushing this green goop into all of the cracks and crevices that I have so carefully carved into my master.  I let it dry and repeat the process several times.  After the basic details are fully filled with mold material, I layer on a thick coating of green goop and add gauze.  The gauze is twofold; it helps strengthen the mold and keeps it from tearing.  I add in several alternately placed layers of gauze running in different directions.  If I am building a rather large mold like a portal, I will stiffen up the outside edges by putting wooden strips into the mold and even bridging between the legs with longer wooden strips (see Figure 1).  The Mountains in Minutes will turn dark green when it is fully dry. 

   Interior of mold.     

Figure 1. Reinforced mold -- interior and exterior.

After it is fully dry, carefully work the mold off the master and then carefully clean the leftover plaster pieces out of the mold with a brush and water.  Your mold is finished and ready for reproduction of your master (see Figure 2)!

Figure 2. Masters and molds.


I like to use a flat surface for my mold to sit on when pouring plaster.  I use a flat board that is bigger than my mold and place the mold in the center (see Figure 3).  One thing to watch for when pouring duplicates is for the mold to have unsupported edges, as they will sometimes bow out.  You can eliminate that by using sand to support the outside edges of your mold while pouring.  Simply put sand all the way around your mold almost to the top edge of the mold. 

Figure 3. Pouring into a mold.

Next, coat the inside of the mold with a mold release.  I use a concentration of soapy water to coat the inside of the mold and apply it with a brush, and I have seen others use cooking PAM with the same results.  In any case, you want a mold release.  Mix your favorite plaster again and pour it in.  Gently tap the edges to drive out the bubbles and let it harden.  Once it is hard, plop the duplicate out of the mold and let it dry.  Check it over and if it is satisfactory, paint it with your favorite paints and install it on your layout.  Now you have something to impress your modeling buddies

Mold and Master Storage

Sooner or later, your mold will grow old and lose its detail.  You can make another one if your master is still in good shape.  By putting the master back in the mold when you are done making castings, you will protect the master and the mold from damage during storage. If treated right they will last for years.  

Good luck and have fun.

Click ARTICLES to read other techniques for model railroading.

 Sierra Scale Models is owned and operated by Dave Barron, an NMRA Master Model Railroader. If you have questions about the products, send a message to David Barron or call 813-907-3343.