Hello Richard, One reason I like the finer soda blasting is because the coarse media types tend to pit smooth surfaces such as caliper bores or carb bodies which are critical...many times the naked eye can't see the pitting that takes place that could damage the O rings. Most likely the bores would be masked off but sometimes the sheer air pressure pulls back the masked areas. In any case, the calipers will end up being painted after blasting anyway! Take care.
First, there is a huge difference between an aluminum carburetor and a cast iron brake caliper.
As for sand blasting calipers, what pitting could possibly damage the O rings? You don't media blast the calipers with the O rings still left in.
As for pitting the bores of cast iron calipers when you sand blast the outer portion, I don't even think it is possible to do so, even if you were trying to do so. And, even if it is possible to "pit" the bores, it would still not have any effect on the proper functioning of the calipers.
The brake fluid seal that takes place in the caliper is between the O ring seal in the caliper body and the smooth surface of the piston, not the caliper bore. The smoothness of the caliper bore that could possibility be affected by sandblasting is not critical and has no effect on the "seal" of the brake fluid.