Seems to be the correct method as specified by acrylic manufacturers. Check out the Finishing Guide PDF. Perhaps try the coarser grit and then the finer grit over the same area as they specify.
"All methods of sanding will result in the removal of machining marks, and produce a matte finish. The choice of hand, palm, random orbit, disc, belt, or drum sanding, depends on the quantity, size and shape of the acrylic sheet. Like sanding wood, work from coarse to fine paper. Use light pressure, and keep the part or sander moving to avoid heat build up (See Fig. 11). After sanding, the edge is ready for buffing or flame polishing."
Should I use 'wet or dry' sandpaper? Because I see alot of modders use this type of sandpaper, but I don't know why. Sometimes they use water and soap to sand something down, sometimes they don't, what's the difference between these two methods?