Narrative Forge: Greenstuff Casting 101

In our Fabricator General series, we talk about hobbying techniques and tricks, and ways to build, model, shape, and fix everything, with an eye toward information for both beginners and veterans. This week, BuffaloChicken is back to talk about casting your own greenstuff objects.

Last week I talked about finding, collecting, and storing bits. This week, I’m gonna talk about making them.

Of all the tools available to hobbyists, there is one in particular that’s magical in its utility and should be a mandatory part of any converter’s toolset. It goes by several names (Instant Mold, Oyumaru, Blue Stuff), but the product is essentially the same – a silicone which becomes pliable at relatively low temperatures and only sticks to itself, making it invaluable for making quick and simple molds. By using Instant Mold or any of the other equivalent products you can replicate complex bits, designs, and textures with ease.

Don’t let bright colors or different languages put you off – these products are essentially identical.

This technique is great for any number of things – here are some examples:

  • Replicating a custom-sculpted Space Marine Chapter icon for an entire army’s worth of vehicles.
  • Reproducing your favorite helmet for an entire homebrew Guard Regiment instead of spending a fortune on bits.
  • Molding fur or scale texture off models or toys to use for custom capes and banners.
  • Push-molding the entire front half of a model for easy scenic base casualties.
  • Taking ornate details and transposing them onto another model, like lifting a wall engraving from a Cities of Death building and putting that design onto a Sisters of Battle Tank.
  • A million other projects – your creativity is the only limit!

Years ago I painted a rare and expensive Genestealer Patriarch on Throne model on commission – while I had it, I took the opportunity to make press-molds of the intricate throne details. Years later, I made this cast of the Patriarch design to use as the banner for a counts-as Acolyte Iconward.

I’ve been searching for some time for a simple online tutorial to share this magical tool with people. Despite much searching, however, every tutorial that I came across seemed to share the same problem: they made things too complicated! Using Instant Mold is a simple joy, and all the online tutorials I found kept trying to add needless steps, like building LEGO frames.

So as the old adage goes: if you want something done well and are too poor to pay someone else to do it, do it yourself. So here you are, dear reader – the simplest and most wonderful guide to Instant Mold you will ever find.

In this article I’ll be replicating the cockpit from a B-Wing starfighter from the X-Wing miniature game, so that I can do a conversion using that same cockpit without having to chop up and destroy a ship.

Step 1: Inspect the Instant Mold

Instant Mold, like most other pleasurable objects, should be firm yet slightly rubbery to the touch.

A robust and rubbery hunk of unlimited hobby potential.

Good and flexible, but always springs back to its original shape

Step 2: Boil the Instant Mold

Just wait for the water to start bubbling, and toss the Instant Mold in. Let it bob around for 20 seconds, then scoop it out with a spoon or tongs. Squish it around – if it still has hard parts, flatten it and toss it back in for another 20 seconds. Repeat as necessary until it’s uniformly pliable.

Step 3: Squish the Instant Mold

Try bending the Instant Mold like you did in Step 1… it should squish now!

Simply form the Instant Mold around the object you wish to replicate. To remove air bubbles, roll your thumb along the Instant Mold as you press it into the object – this will press the material into deeper crevices while forcing any trapped air out. NOTE: If you are making a simpler stamp mold, there’s no need to wrap the entire object – just put it on a clean flat surface and roll the Instant Mold over the top of it, being sure to force out any trapped air. This type of mold is easier to make and use, but only works for objects with a flat and featureless side.

Step 4: Cool the Instant Mold

Let the Instant Mold cool. You can run it under cold water, toss it into the fridge, or simple relax with a good book and wait. The wait shouldn’t take long – 10-15 minutes should be more than adequate in your average room temperature – but it’s always better to err on the side of waiting too long. Trying to remove the mold too early can stretch and deform it, as it will still be pliable, so I recommend poking it with something to guarantee it’s fully hardened before you move to the next step.

Step 5: Cut the Instant Mold

After the Instant Mold is fully hardened, take a very sharp hobby knife and carefully cut the mold to release it. You want to cut as little as possible and leave a rubbery “hinge” along one edge, so the mold can open and close like a clamshell. In the photo above I only cut up to where the knife is on both front and back, so the entire right side remained intact. You most likely don’t want to damage the bit you’re replicating, so you can pinch and stretch the mold as you cut to avoid cutting into the original object. NOTE: This step will not be necessary if you made a simple stamp mold. Just lift the hardened mold up and you’re ready to go!

Step 6: Cram the Instant Mold

Roll up a ball of the material you plan on using and shove it into the mold, using a toothpick or other tool to make sure you force it into crevices if necessary. Make sure the “clamshell” mold closes all the way – the larger the gap between the mold halves, the larger your “mold line” around the bit will be. Having some material “ooze out” where the mold joins is normal and easy to clean up later.  Greenstuff (or the equivalent product, like Kneadatite Blue/Yellow) is most common, but anything that air-cures and doesn’t generate heat will work. Brown Stuff is best if you’re replicating a bit with sharp edges or which you want to be less floppy (like swords), while air-drying clay is a cheap alternative for larger objects. Many resins heat up as they cool, and we know what heat does to Instant Mold, so don’t use them!

Step 7: Let the Material Cure

Remember that tumbler of scotch and book you left by the fireside when you checked to see if the Instant Mold had cured? Return to those now and wait. The length of time will depend on the type and volume of material you used to fill the mold, so always follow manufacturer’s instructions. For greenstuff I typically leave the mold untouched overnight.

Step 8: Remove the Instant Mold

This is the moment of truth – open the Instant Mold “clamshell” and pull out your prize! If you’ve done everything right, you should pull out something like the photo above. A wavy “frill” mold line is normal.

Step 9: Trim Off Excess

This is the same recast part after some trimming and cleanup. Be sure your hobby knife is extremely sharp – greenstuff is rubbery, and a dull blade will sometimes bend floppy bits instead of cutting cleanly through them.

Step 10: You’re Done!

This shows the original bit (left) and recast bit (right) after painting. The left of detail is almost identical. Also note that the original model is entirely unharmed – despite being painted with water-based acrylic paints and not sealed at all, the entire recasting process left no damage or residue on the original. Also, now that you have the mold, you can re-use it as many times as you want! When you don’t need it any more, simply toss it back in hot water and use it for a new project.

Other Examples

I can hear you saying “I dunno, BuffChick… this seems like some kinda technique for X-Wing nerds. How can you prove that it’s also useful for cool, sexy 40k players and their much more expensive minis?”

Let me show you.

I only had a single pair of legs for two scorpion upper-halves, so used a simple stamp mold to replicate the legs. Now this lil’ buddy is ready to terrorize a Deathworld Forest terrain piece or model base!

This extremely weird dwarf head seemed perfect for  Squat servo-skull, but I didn’t want to buy another $35 character model to get a second one. Instant Mold to the rescue.

Here’s the servo-melta ready for barrel-drilling and paint. Although the recast bits look a little rough before priming, they look as good as the originals once painted.

Great look at the detail possible with an Instant Mold recast.

Turn any old toy into a genuine 40k terrain piece with a tasteful smattering of skulls and cogs. This stamp mold was pulled off a Cities of Death building.

The torso for this jungle Chaos Spawn didn’t need to be perfect, so I made an Instant Mold copy of a Deathwrld Forest bit and filled it with green Sculpey oven-bake clay. I then opened the mold back up immediately, dropping the clay onto a tray and baking it until hard. This is perfect for bits which take up a lot of material and where a little deformation won’t be noticed – because Sculpey costs less than greenstuff, it’s a good way to save that more valuable material for other projects.

What about recasting bits with undercuts? As you can see here, the rubbery nature of Instant Mold allows it to capture minor undercuts like the protruding lip in front and shallow barrel holes. Even with extreme undercuts, you could simply cut the mold in a few places to remove it.

The same bit cleaned up and in place. After priming and painting, it should look just about the same as the originals. Any loss in depth can be mitigating with careful painting, shading the recesses to look darker and deeper than they actually are.

Go Get Casting!

Now that the shackles of bit acquisition have been loosened (if not entirely shaken off), get to it! If you use this technique successfully and want to share your hobby success, we’d love to see it. Post something in the comments below or shoot us an email at

You can always find BuffaloChicken’s latest model photos on Instagram.