Working with foam

Article Synopsis

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This article is about using foam to build structures of various kinds. index.php?title=Category:Works In Progress
Under construction/modification by DErik (talk) 11:54, 3 December 2023 (PST)

You may have heard people talking about how foam can be used to make mountains, walls, even buildings. This article is about how you can do the same. It will describe the materials that are used, provide instructions on how to use those materials, discuss some of the pros and cons of using different materials and working with them in different manners, and provide some known sources where the materials and tools can be found.

If you are skeptical, you have good company. But be aware that foam structures can be as durable and resilient as most other building materials, such as wood or plastic. Furthermore, foam can be made virtually impervious to weather conditions. And it can be used to build those things mentioned above, and perhaps a bit more. And while you may think that working with them is a bit difficult, well, in a sense you are right. But, as with most other things, proper procedures, patience (lots of it here sometimes), and attention to details will prove most valuable.

Materials

Tools

You have probably heard some of our members mention Hot Wire Foam Factory (HWFF). The tools they provide are very good for cutting and sculpting foam. Now, they are aren’t exactly cheap, but not too expensive either. And best of all, the Rose City Garden Railway Society has purchased a tool kit that is available to any member. Just contact the Yardmaster (email link). For more information about them, refer to HotwireFoamFactory.com and be sure to visit the FAQ pages at https://Gallery.HotwireFoamFactory.com.

As with most tools, there are substitutes that may be used, and there are some drawbacks, but, in my opinion, their tools are the best available. You might consider using a regular knife, or X-ACTO, or even a fine toothed saw, to cut the foam; but this is strongly discouraged. All these tools will tend to rip the foam rather than cut it, leaving jagged edges and possibly holes in your foam. And they will leave a lot of fine residue all over the place, which is very difficult to clean up.

Have on hand lots of disposable mixing cups, stirring sticks, putty knives or sticks to spread stuff with, paper towels, and disposable brushes. A sharp, thin serrated knife is handy for inside cuts where hot wire cutting isn’t practical. As you will deduce from the discussion below, working with this stuff can be very messy. So be sure to quickly clean any tools.

Foam

The type of foam to use is important as well. Regular styrofoam is not a very good material. It will “melt” when most paints and adhesives are applied to it. It is not that strong either, so a slight bump against it may dent or even break it.

There are two other kinds of foam that are strong enough and can be painted or coated to provide very good weather protection. The first type Extruded Polystyrene (XPS) board. This is frequently used as insulation. It can be found at most hardware stores. There are a few types of this: a pink version, a white version, and even a blue version. The pink version is somewhat stronger and the better choice for outdoor structures we intend to use it for. It comes in various sizes: 1/2”, 1”, or 2” thickness are usually available; 2’x2’ and 4’x8’ sheets. The larger the sheet, the cheaper, as you would expect.

Another type of foam is urethane foam. [Need more details on this from Mike and Warner.] This is even stronger than the XPS, almost as strong as hardwood. It is as easy to work with as the XPS. However, it is a bit more expensive, and XPS is strong enough for most things.

Foam examples

Pink insulation foam

Pink insulation, stronger urethane foam, and 2" thick piece cut with a hot wire.

Adhesives

Adhesives are crucial to construction, and working with foam is no exception. For foam, you need one that will harden and hold layers of foam as well being able to be cut through with the hot wire tools. Some adhesives, believe it or not, dry so hard that it is difficult to cut through them with a knife, and the HWFF tools won’t melt through them either. Others may melt the foam, becoming totally useless.

The adhesive the club members have found to be the best is Foam Fusion, available from HWFF. It is a chemically drying substance, not air drying like white glues. It hardens well, holding layers together well; and it can be cut through with the HWFF tools.

Urethane adhesives, such as E6000, Sikaflex, or Loctite PL3x, cannot be cut through with the HWFF tools, so using these is highly discouraged.

A dual-purpose alternative is Seal Grip (Gripper) by PPG; this is available at most hardware stores as well. It can be used instead of the Foam Fusion mentioned above, and is somewhat cheaper. For most things, this is probably the best choice for value.

Adhesive examples

Two adhesive products from Hot Wire Foam Factory.

Seal Grip used instead of Foam Fusion.

Paints and Coatings

Exterior Foam Coat (EFC), also available from HWFF, makes projects solid and durable - like stone. It is a cement like powder that mixes with water or latex paint. It dries strong and gives a sone-like texture over shaped styrofoam. It can be mixed with paint or painted after it sets up. There are several additives that can be mixed in for various properties to adjust curing and flexibility.

Acrylic or flat exterior latex paints, or even dry tempera paint pigments can be mixed directly with the EFC to establish a base rock or ground color. You should slowly mix the EFC with water (or paint and water) to a thickness like pancake batter - runny but not soupy. Thinner mixtures are easier to spread, but will not be as strong when set. Because the EFC is very white, a resulting blend of paint or pigments will naturally be quite a bit lighter or brighter than you might expect. It is recommended that you try a small mixture of it on a sample piece before doing the whole project; but if needed you could recover the entire project with a second, or third coat after adding more paint to get the color right. Pastels and chalks can be applied after EFC hardens, then sealed with a clear acrylic matte spray. Typical earth tone colors are useful: yellow oxide, raw sienna, burnt sienna, burnt umber, hooker green, and Payne’s gray.

For a realistic sand or dirt texture, apply the material directly onto any wet layer of coating. If greater adhesion is needed, perhaps for apply small gravel or something like track ballast, spread artist matte media, decoupage, or Titebond III glue over the surface before sprinkling the sand or whatever over it. Sifted dirt can be applied over generous wet coats of Flat (not gloss or satin) exterior latex paint. Sanded grout can also be used but the color are fixed and don’t blend well. Use spray bottles of water to lightly moisten surfaces before applying dirt or sand to a painted surface.

Paint examples

Use small paint samples from paint stores.

Rust-oleum works well for stone finish.

Sample Structures

Here are some examples of what can be done with foam.

Finished structure examples

The base is made from foam.

The edging at the top of the tunnel is a separate piece, lamented between two other pieces.

The sand was sprinkled on while the coating was still wet.

Hints and Cautions

Foam board has a grain. You might have to look carefully at the board to see it. Cutting along the grain is, obviously, easier than cutting across the grain. But be care to cut where you want regardless, as the grain may not follow your intended path.

Straight cuts, even with the grain, are had to make perfectly perpendicular. If perfect cuts are needed, use a guide, such as a ruler or wood board clamped to the foam, to guide the hot wire tool. A thin kerf blade on a table saw might also work, though be aware this will cause a lot of foam dust, and may at times create a jagged edge.

Exact small cuts (like window openings) are best done with a long sharp utility knife because, due to melting at the cut line when using a hot wire tool, it is sometimes hard to judge exactly where the cut is. Adjusting the heat of the tool very precisely and/or using a wood template will help. Or, if you can, heat the knife so it melts instead of cuts to avoid tearing, ripping, or otherwise ruining the cut itself.

Don’t try to use white glue, like Elmer’s; it won’t work with XPS or any styrofoam, because it doesn’t adhere to foam materials. And white glues have to air dry; that will not happen between layers.

Scratch or scuff up the surfaces, both of them, that are to be glued together. This will give the glue something to flow into and anchor as it sets.

Allow time for surface gluing to set. 24 hours is best with Foam Fusion or Seal Grip. Use clamps or weights to hold layers together overnight. And be sure to prevent shifting of the material. Adjacent layers being glued together will shift while curing unless you do something like sticking toothpicks through adjoining pieces of foam, at differing angles, to prevent this. If toothpicks don’t work, you may need needs (like finishing nails) or even screws - like gluing pieces of wood together and using screws to permanently hold the pieces. Just don’t put screws, or nails, where you might have to cut later.

When mixing paint with white Exterior Foam Coat, you can use household exterior flat latex from any hardware or paint store. It’s handy to buy sample sizes for model work and have lots of colors. As mentioned, EFC is white, so use a darker shade of paint that you think you might need. A base layer of EFC that is not white might be preferable in case the next layer allows the base to show through. For example, you might add a small amount of black dry tempera powder in the mix to create a neutral stone-gray base color or use a dark shade of “earth” color latex paint so the base layer will look like dirt or grass.

Gripper can be used instead of Foam Fusion. For one thing, it is considerably cheaper, and readily available at most hardware stores (like Lowe’s or Home Depot). It also can be used as a “filler”, to plug up a hole or depression in the foam.

The Foam Coat is a powder, and will make a terrible mess if shaken or stirred. WEAR A MASK. And wear disposable gloves, like nitrile gloves that are flexible and tactile, yet provide complete protection against corrosive materials or those, like powder, that are hard to remove.

A tip I got from a clinic: When storing paint for an extended period of time, put a thin layer of normal water over it before closing the lid. And be sure not to shake it much after doing that. This will provide a “seal” to prevent “skinning” of the paint and a source of moisture to keep the paint “fresh”. Without it, paint, even in a well sealed container, will tend to dry out and have to have water or something added to it later anyway.

Authors and Contributors

• Mike Fisher and Warner Swarner
• Author and editor: Don Erikstrup (DErik (talk) December 2023)
• Others: Please comment on this using the “discussion” tab above or send an email to MRT SIG . And contribute additional information here and in other articles.