Model Making Quick Tip: Polyurethane Foam

Hey folks, Dave here with another quick model making tip from Thunder Mesa Studio. So, what do these cacti, this rock spire, that water tank, those bridge footings, these stone steps, and this chimney all have in common? Well, yes, they can all be found on the Thunder Mesa layout, but the more important thing they have in common is that they are all made from the same basic material. This stuff, high density polyurethane foam, sometimes called gold foam, prop foam, or simply carving foam, it comes in several different shapes, sizes, and densities. Shouldn't be confused, however, with polystyrene foam. This is extruded polystyrene, usually comes in pink or blue, and it's the stuff you find in the insulation department at your local home center. Also, don't confuse it with white beadboard styrofoam. Both of these foams are handy and have their applications in model building, but are much, much more difficult to carve than polyurethane foam. If you've never used gold foam, imagine green floral foam, except this is much denser and stronger. Generally speaking, the denser the foam, the harder it is to carve, but the better will hold fine details. This is a 10-pound foam, my personal preference. It's a nice happy medium between strength and carvability. A consumer version of this stuff called balsa foam was marketed to artists and crafters for years by the American Art Clay Company, but it has recently become a little bit hard to find. Fortunately, there are other manufacturers offering very similar products, and those can be found from online sellers like Amazon. I'll put a link down below. Gold foam is very easy to work with. It can be marked, cut, carved, scribed, sanded, and shaped with a variety of hand tools. My favorites to use are a simple hobby knife, razor saw, sandpaper, basically any tools that you would use to carve wood you can use on gold foam. And yes, you can cut big chunks into smaller chunks on a small table saw or bandsaw. One tool you definitely don't want to use with polyurethane is a hot knife or hot wire cutter like this. For one thing, it has a much higher melting point than polyurethane foams like the blue or pink or white stuff you might be familiar with. Plus, it'll give off some nasty fumes if you melt it, so you really don't want any part of that. My most recent project with gold foam was this little stone outhouse. Let's take a look at how that went together. Now, I'm building an outhouse here as an example, but obviously the same or similar techniques can be used to model whatever you want. The first step was using a razor saw to cut a block of foam that was a scale 4 by 4 by 8 feet tall. Then, I used a sanding block to clean up all the edges and make sure everything was nice and square. I used a blank of the door to mark its position on the foam, and then used a hobby knife to carve away material until the door would fit recessed within the walls. This isn't hard to do, but requires a little time and patience to keep things square. I used my sanding block to round the corners just a little bit, then measured and marked the top wall so I could add a pitch to the roof by cutting away material with the razor saw. Just a little more sanding was needed to flatten the surface for the roof. To begin carving the stones, I used a hard lead pencil to place guidelines about six scale inches apart. Then, I used a pin vise with a small nail in it to begin carving the stones themselves, doing my best to keep the shapes organic and random while staggering the joints in proper masonry fashion. Bricks can be done in just the same way by keeping the shapes measured and uniform. Obviously, my outhouse was modeled as a solid structure without any kind of interior. To make a hollow structure, you could carve away all of the interior material, or cut and carve four separate walls and then glue them together. PVA glues like white glue or yellow carpenter's glue work well with gold foam, though expect longer drawing times than you might find with wood or paper. By the way, it's a really good idea to have a shop vac handy when you work with polyurethane foam because it does make a bit of a mess. It's safe to use rattle can paints with gold foam since it won't react and melt like some foams will. I used a red oxide primer on my model, then I filled the grout lines with spackling compound. This is an old, reliable technique that works very well with brick walls, too. As the spackle sets up, you can wipe away the excess with a wet paper towel, leaving only the grout lines filled. With the spackle dry, I used a small brush to pick out and add variety to individual stones with a selection of earth-toned acrylic paints. To tone down the white spackle and tie everything together, a very thin final wash of dark brown mixed with black was brushed over the entire model. I made a door from some scribed basswood and scale lumber and glued that into place, followed by a shingled roof trimmed in matching colors. Some final dry brush weathering and the model was ready to go on the layout. And that is just a quick example of what can be done with this versatile material. It's not perfect for everything, but for stonework and natural forms, polyurethane foam is hard to beat. I hope you'll give it a try. Thanks for watching that quick model making tip and thanks to Thunder Mesa Studios patrons for making it possible. Don't forget to subscribe if you'd like to see more. Adios for now.