One addition to what Chip has written is on aircraft engine fan blades, when we blend out nicks in the blades (which are subject to massive centripetal forces), the "blend" should be 4x the depth of the nick. So if your blending a blemish on a rod etc as Chip was, measure the depth, say 2mm, the blend would be 8mm with the deepest section being where the original blemish was.

I'd start with a weathered block (as BMW used to with their F1 blocks), then destress it as Chip has, wirelock all the fastners etc, perhaps it won't make any noticable difference for your average fast road engine, but its attention to detail.