Yeesh, did I say he should stop painting?
I'm sure once you get to display-level talent (and enjoyment), painting metal isn't more difficult than any other color. But that wouldn't explain the articles and other posts I've read trying to learn to paint, nor my personal experience, with "clumpy" metal paints. (EDIT: Will try Vallejo Air Model Silver, which others have recommended to me!)
*Anyway* Dragon's post reminded me of this highlighting and shading article that also mentions turning the figure upside-down, as well as washes. I'm not sure how well it applies to this particular model and if it matches OP's skill level (I'm still wrapping my head around it), but I found it informative.
http://blog.brushthralls.com/?page_id=2650
Also, in order to shade metals (same principles as other colors, of course...) working at night, I'll turn off all the lights in the room, have a strong light behind me, and paint in the shades on the figure. Realistically, I only do this for metal, shading with a darker metal color (eg. mixing black with a metallic). If you do it right, you can't tell (annoying). If you do it wrong, you'll notice it (very annoying). Do the same with highlights, painting the highlight on whatever reflects. Another trick is that I'll use a black Micron 005 pen to blackline a small area at a time, followed by massaging in the ink with a damp brush. This includes that little shadow underneath studs in the armor.
Chibi, let me know what worked for you and *why* it worked. Painting is an art, not a science (rimshot) but I'd like to better match which technique to use for which specific situation -- like yours.