Main Case Restoration

John, the use of sand blasting on any part that is part of the oiling system is asking for a future problem. ~Even with careful masking & judicious cleaning there always remains the chance that some sand will remain in bolt holes, cracks, under flanges & other pockets.
~While there are some restorers that do sandblast the main case, most don't, including myself.
~I have found that paint stripper, followed by a power wire brush & smaller rotary brushes in a die grinder or drill & even small hand type wire brushes will do a great job of cleaning the main case. An air powered pneumatic needle scaler is used for the inside corners & deep pockets or other areas that are difficult to get a wire brush into.
~After the wire brushing operation I usually treat the outside of the case with a mild solution of Phosphoric acid to kill the rust in the pores of the casting then give it another hit with the rotary wire brush then prime with epoxy primer.
~See the attached picture for a 37 "A" case that was done with power wire brushes, needle scaler & phosphoric acid.