A word of caution regarding sandblasting, depending on the sandblaster size and experience level of the operator, a flat piece of sheet metal can be made into a "Pringle" shaped disaster due to the heat(friction) involved produced by the air pressure/volume of sand. Proceed with caution. I'd use a scraper to get the heavy stuff off and follow up with a coarse Scotch Brite Pad. Maybe soak it in a Coke bath for a day.