A common error is to scale the media to the workpiece: big part, big shot. The relationship that matters is between the particle and the feature. Media cannot clean or peen a recess it cannot enter, so the smallest internal radius, groove or hole in the part sets the upper limit on grain size.

Coverage sets the lower limit in a different way. At a given mass flow, smaller particles mean more impacts per second and therefore faster coverage, but each impact carries less energy. A peening intensity that cannot be reached with fine media has to be reached with coarser media and fewer, harder impacts.

Finish follows size directly. Fine media produces a fine, even, satin surface; coarse media produces a visibly textured one. Where the blast finish is the final appearance rather than a preparation step, size is an appearance decision as much as a process one.

Where the two limits conflict — a deep recess that also needs high intensity — the answer is usually two operations rather than one compromise size.