I usually use a 5 inch random orbit sander for the sanding. You may be able to start out with a belt sander, but be careful because if you remove too much material too quickly the acrylic will start to soften and get gummy.

• Start with 60 or 80 grit, and spend a while sanding each edge of the assembled case. Keep rotating the case from edge to edge until all the layers in the sandwich have reached the same plane. Often my front and back sheets will be sticking out further than the core, but this will depend on the laser and how well you aligned the front plate when you glued it to the core piece.
• After you have all four edges flat, then do the four corners, rounding them over and blending the edges of any side plates that may be present.
• Start rounding the corners between the side edges and the main faces of the case. Try not to sand the faces much since any scratches there will just mean more time at the finer grits getting the gouges out.
• Move to 120 grit, and resand all the surfaces you did above until the scratches are gone. Also do the main faces now.
• Soften and blend the corners where two edge faces and a main face come together. Careful not to overdo it.
• Go to 220 grit and go over the whole thing
• Finish up the sanding with 320 grit over everything

Sand the Crap out of It