ignore the many, many node lines in this image.  they're from my face shader, which uses layers of makeup, material based sRGB correction (i don't have PP), and other features.  and i know the metal doesn't include conservation of energy, or any other more realistic effects, but i wanted to show how to use a very, very simple metal material.  but i did want to show how this method allows you to use a totally different type of specular than the skin.  not that you couldn't bagginsbill's way. 

on the grouping tool:  actually, i've used it.  a fair amount, even.  i hate it with a passion.

it took me longer to make the image showing how to do this than it did to make my mask and set up my material. 

it took me longer than either to select exactly what i wanted in my very simple clothes to do some testing earlier today.  and the topology of my clothes is way simpler than a Version 4 face.  someone else might be totally faster, but for me, that way would be a waste of time.

i can save this material as a map to use with any V4.  very light-weight file.  to save the grouping, i'd have to save a new character file.  not at all light-weight.

i can blur the mask to make the transition softer.  i can shred the mask's edges to distress it (should my metal nosed character enter a conflict).    if i used grouping, i'd have no options.  and using grouping i have a truly hard line.

i'm not saying the grouping way is worse, because for some people it would be better. it just seems like skill at using that tool is  uncommon among average users.  and i've read tons of posts saying how awful it is by really skilled modelers.  since my own experience has supported this, and it's a very limited and fixed solution (which is something i always end up having a problem with), it's not the solution i personally would recommend.  it's great that you and ockham are here to suggest it as an alternative.  just not what i would do, you know?