Whether glass beading, soda blasting, or vapor blasting, all these processes use a media component. Your cases need to be flushed (oil galleys) and threaded holes cleaned prior to reassembly.
Personally, I would send them to Nils. His work is excellent, his prep and protection are second to none, and the finish is beyond gorgeous. You still need to use compressed air and some Carb Cleaner to flush the galleys and holes. I say Carb Cleaner and not Brake Cleaner as Carb Cleaners have an anti-oxidant in them that will not discolor or stain your cases. Brake cleaners can.
Tews, see your FIL and borrow some thread chasing taps. Apply a tiny bit of oil, and run them in each case hose. Spray down the tap after extracting. You’ll be surprised by how much media you collect. Be super diligent and the outcome will be superlative.