for something like a brake caliper you need to be carefull. sand is a no-no (far too aggressive) and soda is probably too gentle. Your best bet is glass bead, or media (wallnut shell) glass is a lot finer, so would be the best bet.
ABC stainless in Peterborough are the best by quite a margin for this sort of work. speak to Tim at paneltec, as they do send stuff there from time to time, a caliper could likely be 'lost' with a larger job.
it was ABC that did the fairing sub frame and grab rail on the GPz before they were powder coated.
for something like a caliper going to peterborough (if paneltec dont have anything going there), postage wouldn't be much, probably about £5 tops.