Start by duplicating the subtools you want to weather. Inflate them slightly using the deformation palette.
Then dynamesh the new subtool.
The reason for this is because the mesh might not have polygons spread across it equally, so dynameshing resolves this!
Use a combination of brushes to chisel away at the mesh to reveal the one underneath.
I use Crack Orb brush pack - get it for free here
Weather down the hard edges. Use the layer brush to add some complexity and give the appearance of chipped-away paint.