Step One: Cutting
I have used hot wire blades, exacto knives, and others in the past. What I use to cut my terrain pieces out is a small utility hacksaw. This allows me to cut down the shape and form of the foam very quickly. I find it fast, but a little messy. I generally do my cuts in and over a large bin for easy clean up.
After the pieces are cut out, I do some ledges and other details. Like making sure models can actually use the terrain (this is my biggest pet peeve about store terrain). This means creating ledges on rocks that we can use in the game leaving room for sizes up to terminator bases along rocky outcroppings that can be used during a game.
Leaving room for rhino size or even land raider sized tanks, is also a must. When people make terrain that does not take into consideration the models it was meant to interact with, it makes for a frustrating game. Nothing like having to dig out models in a piece of terrain you can't get your hands into.
This is where I had made a few mistakes....... Get your models out and place them into or around the piece of terrain. This way you will catch mistakes and see how you can get the terrain to work better with your models. Do not forget terminator size bases, and crouching heavy weapon teams for Imperial Guardsmen. For tanks, remember that barriers need to reach 50% coverage. Please do not forget to take line of sight to and from models using your terrain. The goal is to make it easy to use, with little disagreements about what models can and cannot see.
Not taking the game into account is just bad design, no matter how good it looks.
Step Two: Burning the Foam
The one problem with Styrofoam is getting paint on the surfaces, and making them rigid enough to be used on a daily basis. This is why I treat the surface of the Styrofoam (I don't do the bottom). I have a sculpting tool that I use (in the pic above). This allows me to burn the surface, literally melting the top. It creates a more solid and hard (a little crunchy) surface.
Please do this outside...... or away from others, and wear a mask yourself. These fumes are toxic.
This is simple and fast. Simply scupt the terrain. This is where some of the details in the form are done. You want to make sure every surface that is going to recieve paint is treated here. The more you melt away here, the harder the surface will be, but don't get too carried away, that foam will melt like butter.
Step Three: Paste
Another messy step. Get some flour, add water. You want a heavy, but not molasses thickness to your paste. To thick and your dry time is rediculas, or worse, it will not dry underneath before mold sets in. Please add a decent amount of salt to the paste, as this will help make sure it does not mold. Apply with a large brush over your terrain and let dry. You may need to do this a couple times.
The already burnt surface of the foam will take the paste quite well, and once dry, will form a pretty hard surface. It can still be broke.... as can anything, but it will be usable for terrain. The biggest part is that now the foam will accept spray primer and not melt.
You may need to do a couple of coats. I try and do it in two. (faster drying, and a better surface)
To be continued once I get past Step Three (doing it today). here is pic of me checking my terrain with models.