The rough texture left by the blast media is creating thousands of places for rust to get started. Your blaster guy should have used glass beads, not sand, as his media choice. Now you need to use metal prep (phosphoric acid) to deep clean the metal and neutralize any imbedded rust, before giving the metal a thorough HOT water scrub and an immediate primer coat before the flash rust can form again. Metal that's been pitted by overly aggressive blast media is VERY hard to work with.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!