Hi Alan
Yes it is perfectly feasible.
As Jason suggests, first heat the cast iron to red heat in air, (no flux) to burn off the carbon. Allow to cool naturally and clean with a stiff wire brush. Apply a long life flux, like HT5, and proceed normally.
I wouldn't recommend sand blating. If the sand gets into the surface of the cast iron, you would be in a worse place than when you started. You definitely cannot silver solder over sand!
Regards
Keith