Sandpaper and sanding sponges are used in puppet building to smooth and refine contours — most importantly on foam, where they behave nothing like they do on wood.
Sanding foam is really shaping foam
On wood you sand to remove roughness. On reticulated foam you sand to carve, because abrasive contact tears the open cells away and softens a shape in a way no blade can. This is how a faceted carve becomes a smooth skull, and it is the finishing step most beginners skip.
Coarse grits, not fine
Sixty to a hundred grit does real work on foam. Fine paper simply glazes the surface and removes almost nothing. This is the opposite of the instinct most people bring from woodwork.
Sanding sponges beat sheets
for foam, decisively. A flexible block conforms to a curve and works evenly across it, where flat paper contacts only the high points and flattens the shape you were trying to round.
Technique
Light pressure, broad circular strokes, and keep moving. Pressing hard digs a hollow you cannot undo, because foam is not solid and it does not sand back level.
On rigid materials
Cast resin, cured paper mache and wooden parts sand conventionally — coarse to fine, with the last pass fine enough to take paint cleanly.
Dust
Foam sanding throws a remarkable amount of fine particulate. Work outdoors or wear a mask; this is not fussiness, it is the kind of dust that stays in a garage for days.
The hack: sand the head before you skin it, then check the silhouette against a light. Lumps invisible in the round show instantly in outline — and fleece hides nothing.