This is a fascinating experiment! Using sandpaper as a support for soft pastels and pastel pencils is actually an excellent alternative to premium sanded papers like Pastelmat. The textured grit of the sandpaper acts exactly like a toothy art surface, grabbing the pigment from the pastel sticks and pencils tightly, which allows for rich color saturation and multiple layering without the need for heavy fixatives.
​Your choice of a 400-grit over a coarser 120-grit was smart—higher grit numbers mean a finer texture, which is much more forgiving on soft pastel pencils and saves your fingertips from getting worn down. It’s also impressive that you achieved such a smooth blend using only the pencils themselves; it proves that the grit naturally shears the pastel core to blend layers together seamlessly. Thank you for sharing this practical, budget-friendly art test!"