First, suggest you read through this earlier thread, and then maybe search for other discussions on the subject - sandblasting ?
Bottom line, without regard for anything else you don't ever want to use sand for abrasive blasting. Sand is little particles of silicone, and not only will you push silicone into your metal making it difficult to paint, you can breath silicone dust which is serious stuff - Silicosis. You'll see recommendations in the above thread from guys who media blast regularly.