cmm, is your question:
1. Is there a spec for plating thickness.
2. Is there a spec for how you indicate which surfaces are plated.
3. Is there a spec for specifiying different plating thicknesses on different surfaces.
1. Has been dealt with in some depth before, try searching a few months (maybe a year) back.
2. I've done in various ways but off the top of my head can't think of an explicit ASME spec.
KENAT, probably the least qualified checker you'll ever meet...