Grim’s Dungeons of Doom: Adventures in expanding foam, part 1; Cavern Walls

17 Sep



Today’s terrain building Grim’s Dungeons of Doom nonsense is going to tackle another type of wall piece that is found in many a fantasy dungeon scenario, the cavern. I touched upon this before in my brick modular wall article at the beginning, but this one is a slightly tricky make. It involves a can of expanding foam gap filler…

Now while I have used this before on all sorts of craft and wargaming projects, it seems to fill a hell of a lot of people with dread, and with good reason. Its toxic shit in its unset state, extremely sticky and can be impossible to remove from hands, clothing and surfaces that it shouldn’t be near etc. but don’t let this trepidation of the unpredictability of the stuff, or even the fear of it’s unknown properties stop you from producing some nice looking and durable wargaming terrain for your games!

There are lots of types of expanding foam you can get if you look for it, but I’ll deal with this expanding gap foam, which is easy enough for most people to get hold of if you want to try it out. It’s pretty cheap, sets hard, is easy to carve and takes any sort of paint well that I have used so far in all the years I have done anything with it.

(If your reading this Gorilla/151, I’d be grateful of some freebies in return for this product placement, Cheers!)

So let me guide you through the process, which is a bit intensive compared to the simpler methods I’ve covered so far. By intensive I mean that you need to get your timing right, its not enough to leave the foam to its own devices, you’ll need to be aware of how it wants to behave, and concentrate on manipulating it if you want to it act like you want it to behave. Confused? Don’t worry you’ll see what I mean.

For this you’ll need to work out how high you want the wall sections. Once you do this (which I advise to be roughly the same for all your wall sections for uniformity) stick to it and don’t deviate. You can vary the height by creating sub sets that are slightly different from your usual ones, like ones that form feature rooms etc, but we can deal with that later, for this we are making modular pieces like before.

Get some card and make a trench.  Referring to the picture below, the bottom of the folded card will be the width of the desired wall height, while the sides are the width of your final walls. These need to be about half the size of your desired wall height. What was good,was this pizza box was the right size along the box edge,  so it was easy to chop the box up and cut out the section without making it from scratch. Also it makes creating the corner connectors for the modular walls a lot easier (but I’ll deal with them in the next article)

Make sure you get some gloves and protect yourself from the stuff, don’t forget to prepare your surfaces your working on too, make sure to protect them from the possiblities of accidents. It is a thing to be held in respect, so respect it!

While its in its “wet foam” state,, you’ll need something to keep the sides straight (remember these are the top and bottom of the walls, so you need to keep these level while it does its expanding for a bit) This is just temporary as it’ll start to set enough to start making textures. Spread it around and fill roughly up to the edge of the trench, don’t spray too much in, try and scrape it onto something else to form other bits not to waste it (a good idea is to make a few sections at a time) As it starts the expansion, don’t be afraid to knock the expansion back by using a disposable stick or toothpick to pop the bubbles as they form.


At this point when the wet foam stage progresses to a soft hold and it has held a slight film, get a fresh toothpick or whatever, and knock more of it back and create some surface detail onto it. As it starts to dry and set, the foam can be stretched, twisted and manipulated into loads of weird shapes, but try and make it organic to resemble cave walls.


Once it’s looking like your satisfied with the texture, you can leave it to set. Give it a good few hours (3+) so that its proper set and the inside is cured too.

Next, cut the sides (Now the top and bottom) and trim them to shape like the pic below, you can also trim up some of the foam. Use a really sharp blade to do this, as blunt ones will drag the foam and ruin the effect you’ve worked to achieve.


Next, decide how long to make the sections. Obviously, its better to make sets of the same sizes so it’ll be easier to create rooms later, but it’s up to you. Trim the ends, and again make sure your blades are sharp.



At this stage I can’t help but think, these look like some weird rice crispy bar! Make sure the ends are trimmed nice and flat, keep trying them against another wall section to make sure.


Now, get some textured wallpaper and use it to clad the rear and top of the pieces. Plain white anaglypta textured paper is good for this, it adds tons of quick detail to the plain flat sides.




Trim the paper and leave to dry.


Undercoat it by spray, brush etc. Its pretty good at taking whatever paint you can throw at it.


Paint up by working up shades of dark grey up to eventually white, by drybrushing the whole piece.




Add a bit of a brown wash and muddy them here and there, add some slime, moss and other scenics as you like for detail, and thats pretty much it!


These wall sections look pretty good on both sides and who doesn’t need more cavern walls? These would also work for 15mm too, since there’s no detail to distinguish it in scale.

I’ll be back next time with part 2 of expanding foam creation, with corners and doorways to match your now modular cave system for your dungeons…

8 Responses to “Grim’s Dungeons of Doom: Adventures in expanding foam, part 1; Cavern Walls”

  1. daggerandbrush September 21, 2015 at 1:53 am #

    Thank you for this interesting tutorial. I think that stuff is Polyurethane foam? How hard is it when it is fully cured? I saw the guy on Terranscapes use it to fill in a cavity in one of his casts to safe weight and costs.

    I like how the final piece looks like. It has a nice stalactite cave look to it. Maybe you can enhance this look with adding some gloss varnish to make the walls look wet (I think I use “look” too often).

    Liked by 1 person

    • grimace73 September 21, 2015 at 7:49 am #

      Yep, thats the stuff! It’s quite solid when cured, and will take some knocks.
      Yeah, I saw that vid too, its good multi purpose stuff.
      I’ll admit, my pieces aren’t 100% just yet, I’ve still gotta weather them and add some slime and moss, but no doubt that’ll come in the next articles (well, finished shots from all angles)
      Also, don’t worry about using the same words so much, I think everyone is guilty of doing the same from time to time!! 🙂


  2. Ann Wycoff September 24, 2015 at 2:57 pm #

    Yes, these tutorials are all very interesting!


    • grimace73 September 25, 2015 at 12:57 am #

      Thanks! glad your enjoying it! I’m getting the hang of the tutorial lark now, and hopefully by sharing a bit of knowledge, it’ll inspire others to have a go at terrain building. Its also helped me stay on focus with my original intention, and given rise to things I might not normally have done (the book PDF etc) and some great ideas for the future. There might even be a book or two and some products develop from this if my plans fall in, so I think its working out for the good!
      Next article up is more foam antics, with doorways and corners! 😉

      Liked by 1 person


  1. Grim’s Dungeons of Doom: Adventures in expanded foam, part 2: Cavern entrances & Doorways | The Grinning Skull - October 2, 2015

    […] the whole lot is covered, get stuck in there and move the stuff around as it starts to dry (as mentioned in the previous article) Use a disposable stick to manipulate the foam into interesting shapes, pop the bubbles as the foam […]


  2. Grim’s Dungeons of Doom: Adventures in expanded foam, part 3; Cavern features | The Grinning Skull - October 4, 2015

    […] it with a wooden stick and spread it around to form interest and detail as we did before in part 1 and part 2. As you start to work it, you’ll notice that your stick will start to get bunged […]


  3. 4xD: But hey Mr Grim, I wanna’ game my dungeon in 15mm…. | The Grinning Skull - October 5, 2015

    […] WALLS, CORNERS, DOORWAYS AND FEATURES– Cavern walls using expanding foam are easily done as is, or simply change the dimensions to create smaller walls […]


  4. Rogue Stars: Let the ideas start to flow! | The Grinning Skull - September 16, 2017

    […] that way it’ll be interchangeable with the walls, floors and other fantasy scenic stuff (the cavern walls will fit nicely in an Alien planet theme, maybe some alien flora and fauna, but then I’m […]


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