You're better off not asking Leehy since he's less than Helpful.
the quick and dirty explanation is for a proper bare metal (think OG F-104) skin, you're going to need to muck with the Roughmets of the skin on top of the default diffuse layer. Megalax had some great vids on Youtube that I cannot currently find, but if I see any tutorials of comparable quality I will return with them that can go deeper in on how Roughmets work (that is a forum post unto itself).
Depending on how shiny you want the aircraft, I advise you look at the default MiG-15 or MiG-21 files to get an idea. the Diffuse layer main color for the bare metal parts should be a very light grey or close to off-white as you can. (For a beginner, just play by ear using your eyeball mk 1, see what you think works)
The Roughmet for the Baremetal parts should be a Pink to Magenta color (again depending on the reflectivity you're looking for). If you want a more brushed aluminum look, the P-51's Roughmets are also a good place to look for a bare metal, yet dull look.
I apologize if thats hard to follow, again I will return later if I can find a better explanation. And at the end of the day, You will find theres many ways to skin this particular cat, and my method isn't going to be the same way of creating aircraft dermatitis as other people's methods.
UPDATE:
I found a decent tutorial explaining the process of Roughmet material creation