Two comments:
Rug, I would not sandblast a torsion bar or leaf spring because if the sand is sharp enough it could put discontinuities in the surface that would initiate cracks. The exterior fiber is the most highly stressed.
However, shot blasting or peening may have a positive effect.

On the other topic, I have not understood that to be the case with yellow dichromate bolts, maybe that's why the ARP stuff is black? I wanna know.

R.