I'd say chances are you will never get all the sand out before you paint. Can you imagine, paint's going on fine flowing just right and all of a sudden you hit a lit hidden pocket of sand and it blow all over half the car. Now what do you do??? Cry/start throwing things/go kill the guy that told you to sandblast? All of the above?

The body shop blasted the back window area when they installed the body panel below it. I had sand in my trunk floor for years after.