nope its moisture in your lines! I just had the exact same problem last week and the only thing that solved it was buying a refrigerated dryer for the compressor from harbor freight for 400. How far away is your compressor from your supply? do you have a see through dessicant filter at the gun? I cant remember where I read the thread from barry but he said that stuff that looks like sand is actually the iso's crystalizing due to the moisture or something like that but the dryer definitely solved it.
Oh and It wont matter how many filters and water separators you have it will only slow it down but will still be there, trust me I had three dryer filters and a separator and still got water at the gun.