Using an abrasive material to rub the workpiece's surface to make it shiny or in some rare cases make it un-shiny.
Another similar process is called ‘Shot blasting ‘, which create a very similar result as shot peening, but shot blasting use a high RPM spinning wheel creating a centrifugal acceleration to ‘throw' the media onto the workpiece's surface.
There is a very similar process like sandblasting called shot peening. Shot peening also uses compressed air as power, but the ‘media' contains metallic shots or sometimes ceramic or glass shots(0.1~2.0mm diameter).
Sandblasting is widely used as after-treatment of CNC, Casting, injection process, etc. It can be used to remove rust, cover tool mark, increase total surface area for other surface treatment, deburring and smooth sharp edge.