I use the following technique for acrylics (which I got from someone else on the net):
- Don't prime the figure. Clean it, but don't prime.
- Apply a wash of Liquitex Payne's Grey (a dark blue/black grey) to the fig (adding a tiny bit of dish soap to decrease surface tension -- a good step with using any wash on water-based paint)
Finished. You can polish a bit with cloth or a fingertip to get more highlights if you like.
I've also expanded this technique for fantasy and SF figures where I want golden metal, bluish metal, red/pink metal: just choose a very dark version of the shade desired and use the wash technique above.
Finally, for some figures who have a helmet and little or no other metal armour (especially footmen with bascinets, sallets, and kettle helms from the 15th century), I prime the figure, then file the paint off carefully on the helmet. This works especially well if the helmet is a bit oversized anyway. You get a really shiny metal helmet, and the rest of the figure is primed (and so the paint will stick). The spearman with the red shield has his bascinet filed to base metal; the spearman beside him with the crossbow on his shield has a bascinet painted metal-coloured.
I've used the same scrape-down-to-base-metal technique as an easy way to add a precise metal rim to a shield or edge of a wagon-wheel. The silver decorative detail on the warg-rider's shield is done with this technique. With a dental tool it is possible to scrape the rivet-heads on a figure with brigandine armour so they are unpainted and shine. Works pretty quickly, even on 15mm figures.
Of course, I'm not sure how important it is to have shiny rivets on the brigandine armour of a 15mm figure, but there you are...