Model building using polystyrene foam affords the user with an inexpensive lightweight alternative to the use of wood as a modelling material, all while still allowing the use of woodworking tools to shape the foam. After shaping, as with wood, the final surface of the foam tends to be flawed. The foam shows the signs of cutting and shaping throughout the object, making for a rough-looking surface texture. Removing these signs requires sanding of the foam surface. Unlike wood however, sanding can remove thick layers of material if the correct sanding surfaces aren't used properly. With the correct usage, though, you can create a smooth and even surface on your foam that's capable of displaying whatever level of detail you choose to carve into the polystyrene.
Shape the foam before sanding using a series of hand-held woodworking tools such as handsaws, files and rasps. Cut the foam into the shape desired with the saw, then rip away large portions of the foam to generally shape it with the files. Go over the foam with the rasps to further shape the material until you're prepared to finalise the shape with the sandpaper.
Sand the polystyrene foam down further into its general shape using a 100-grit coarse sandpaper to remove as much of the foam material as possible. Run the sandpaper over the foam form, scraping away the polystyrene in layers until only detail sections remain to be finished. Switch the sandpaper whenever the granules begin to become clogged by foam particles to avoid creating jagged edges in the foam.
Use a sanding block pad to shape large flat or gently curving areas of the foam. Avoid using the blocks on sections of the foam that contain small details; the block may flatten detailed areas.
Use fine-grit sandpaper to perform finishing work on the foam, lightly sanding the surface smooth, or sanding into small detailed curves or edges that the coarser sandpaper or sanding block would otherwise remove. Wipe the foam surface with a clean cloth, then run a second piece of scrap foam over the surface to remove any foam residue.
Wear safety goggles and a dust mask while sanding the polystyrene to avoid getting particles and dust from the foam in your eyes or lungs. Cover nearby surfaces with a dust sheet to ease cleanup after sanding.