If you ever have the opportunity to work with sheet insulation foam, I highly recommend it, though, be prepared to get extremely frustrated, to curse the material, and even lash out violently at the structure into which you will have invested so much time and care. It is a marvelous material for working large scale, in that it allows you to make huge pieces without the inevitable weight of wood, metal, stone, etc. However, it also can’t hold much weight. At all. It will crush itself and fall apart. That is, without an internal structure, a skeleton to which it is mounted. After you have figured out how best to create a low-weight internal structure that you can attach the foam to, enabling you to span distances and fill space without it ripping or denting or snapping(and on that note, be wary of where you set your piece, as it will take a perfect impression of the screws/pebbles/sawdust/ sand/ anything onto which you place it. Even if you have perfectly clean surfaces on which to place it, it will still sometimes just flatten that side, or rip it if you place it down at an angle, or pick it up too fast.)
It is best to design a form comprised of many pieces that can be assembled once everything is shaped, sanded, spackled, sanded, primed, sanded and painted(and sanded).
To create larger forms with the foam sheets, it’s best to make it as solid as possible by cutting out layers that when stacked and glued can then be shaped. Hollow shapes can be unstable and callapse in on themselves, at which point unless it was an extremely clean break, it’s nearly impossible to repair. Gluing is the only option that I found to work for large solid forms. The only glue that worked, and didn’t dissolve the foam, was a spray on tacky foam glue, which were applied to both surfaces, allowed to cure for a few minutes and then carefully pressed together and allowed to dry for 5+ minutes before sanding. Very carefully. Blowing on the curing glue helps speed the cure, though it is fairly likely you’ll start to feel dizzy from the fumes. Best to just let it sit. And do not get it on your skin. And do not misalign the pieces when you go to put them together. This glue was from Michaels, and was not located with the other glues, but in a tiny recess on the shelf next to the various foam shapes. The only reason I knew to find it there was because I worked at Michaels for a few months. There is nothing similar at any other hardware store or craft store that I could find. It is next to a simple and surprisingly expensive wire “Foam Cutting” device. Do not even consider that as an option for shaping foam. It intentionally uses heat to melt through at an enragingly slow pace, leaving behind a jagged edge and spidery strands of melted foam. In your frustration you will attempt to press a bit harder to get it to cut through faster. It will not work. It will make your cuts uneven, and will bend the wire, which will snap and begin sparking inside of 30 minutes. Before the 30 minute mark, you might attempt to use a sawing motion, which will seemingly work to facilitate the cut, but you will then discover a significantly more jagged and less even cut. Just get the glue, leave the hot wire, and get out. A course jigsaw blade works well, a fine one might melt it. A drywall saw would probably work well.
Once you’ve cut out your basic shapes and glued them together, you can shape it very easily and quickly. But not too quickly. Coarse sandpaper by hand it probably the best way to proceed after you have your rough shape. A slow variable speed palm sander is great too, though if you stay in one spot to long, it will melt, but if you move the sander around too quickly, it will rip. Surprisingly, a dremel with the sanding drum also works pretty well, though it will start to grab and dig in unless to use it in strokes, almost like sculpting with a paintbrush. I was even so bold has to try out using a sanding head on a grinder. If you choose to do so, prepare for the foam to melt a bit with every cut, static clinging foam dust to cover you entirely, even under clothing, and for the grinder to occasionally grab what you are working on and fling it across the room, while simultaneously digging a channel through the surface. Grinding one handed while gripping the piece is difficult, and not always effective. A respiratory mask and swimming goggles will be required. It’s ok if you ding it up a bit at this stage, just stop using the grinder before you get anywhere near your final form. You’ll have to switch to paper, by hand or powered, for final shaping. Coarse paper is better, but will never work for a metallic finish, and once you get close to your final shape, it will suddenly seem less convenient, and more perilous to use the coarse paper. Finer paper will be needed. Though be ready to shift your sanding style a bit, as the finer paper will melt the foam much more readily. Again, hand sanding is best. for the final sanding-shaping stage, you’ll want to use a small piece of sandpaper, 220 grit or higher, and work in small patches. Long strokes at this stage can rip the surface and leave grooves. Any creases in the paper will catch and dig in.
At which point you will have to spackle it. But you will have to spackle all the grooves of the layers, and the 100+ other little dents that you didn’t see happening when you were working on the other surfaces. You’ll pretty much have to spackle the whole thing. Then you’ll wait for it to dry and sand it again with a finer sandpaper. Then paint it with a water based primer. Then sand it again with a finer sandpaper. Then paint another layer of primer. Then sand it again. At this point it should be pretty smooth, having taken off all the little burs that form when you paint it. Hopefully you didn’t press too hard while sanding and rip the paint off, leaving a new spot to spackle and sand again. Hopefully.
At this point you can do a very thin layer of spray paint, but don’t let it puddle at all. If it does it will melt through the primer and the foam. After that dries I recommend sanding it again, just to remove any final burs or dust. Spray again. Tadaa! Easy as pie. Just don’t let anyone else touch it or near it or really ever set it down. Because at this point it is basically the most fragile. If the foam gets dented, it takes surrounding paint with it and creates a worse rip that is then extremely difficult to correct.
If you are doing different colors on the same piece and need a distinct line dividing them, I recommend carving a groove that follows the contour of the color change. Just a razor blade carving a v-pattern works. If you want a rounded groove, then spackle the groove until you like it. Then spray the majority color on the piece, then use a piece of cardboard to block the minority color spray from overlapping. At that point I hand painted black in the groove. Choose whatever color you like. Don’t tape off edges, because the tapes adhesive is stronger than the paints bond to the foam. It will peel off in sheets, and might take bits of foam and spackle with it.
This is all based on the assumption that you would be making a smooth object. If you need a melty molten blobular object with accentuated edges, you can go straight to the spray painting. It looks pretty cool.