Blasting Soda is for removing old paint from sheet metal. Use soda instead of sand as it doesn't heat up the metal like sand does and therefore doesn't cause any warpping. After the paint has been removed you still have to use sand to get the rid of the rust as soda won't get it.
I mess with old cars and do a lot of sand blasting. Blasting sand comes in 5 different grits, for an old hand plane I would use a very fine grit. In Raleigh, Capital City Lumber has sand blasting sand in all 5 grits and sells it for $7.00 for a 100 lb. bag. At least that was the price last time I got some, about 3 months ago.
Bob