If you want to protect the lip during the soda blast, you can use some sort of foil tape and don't aim right on it if you can avoid it. At least it will protect some of it (the edges will probably lift).
Some good foil tapes would be something used for shielding heat or electronics.
Regular duct of masking tape doesn't hold up very well to the blasting pressure, buy more layers would (layer foil tape on top of, masking, duct tape) then cut them all at once.
That's my plan for next time. The time spent in masking the lip would be far less than polishing it.. unless, you can put the wheel on a lathe or something similar to polish.