Jerry, in my mind and my experience, the powder coater goofed. He should have masked off the openings, and nothing should have gone inside. Maybe he blasted first, then masked, then powder? I'm willing to give the benefit of the doubt, just a brain fart, but mine came back clean as a whistle on the inside, with no media or powder either as with gold I would have seen it. And I did look.
We rinsed it with solvent like you did, dried it with a hair dryer, the blew it out with air. Nothing.
As a courtesy to him I'd talk to him. Maybe one of his employees did it and he should know about it. Never blast in an oil tank unles that's what the customer asked for.
All you got out was media? No powder coating inside?