Yes, a pressure washer can be used as a sandblaster!  Just as venturi- action is used to pickup soap in a soap injector it is also used to pickup sand and inject it into the water stream.

A standard duty, or hobby sand blaster, works great for occasional use but can wear out pretty quickly and has a fixed nozzle size.  There is a heavy duty model available which has ceramic wear parts and a replaceable nozzle. The heavy duty unit also has a valve to adjust sand flow.

Both types work in much the same way.  Stick the pickup tube into sand and pull the trigger.  Make sure the sand is dry and of consistent size, otherwise the pick up will plug up.  Specific blasting sand is best but bagged sand, available from a building supply yard, is a lot cheaper and can work equally well. Make sure it has been screened down to a size that will not clog the system up.

A couple of warnings. One..... high pressure water, mixed with sand is very aggressive and will destroy many surfaces.  Be very careful not to point the stream at yourself, or anyone else.  Two......... be prepared to use a lot of sand.  Wet blasting is not like dry blasting.  You only get to use the sand once.