3 step rust
-Base coat black
-Dry brush your metallic color
-stipple on Fiery orange (or similar color)
This method is okay for table quality minis. If you want something better use the 9 step method
9 step rust
-Base coat black
-drybrush on tin bitz
-light drybrush of bolt gun metal
-Stipple on scorched brown
-layer on stipple of dark flesh
-stipple on a layer of fiery orange
-wash on either a sepia tone or a brown tone (I prefer brown) Argax earthshade I believe is the name of GWs new brown wash.
-Stipple on an addition layer of fiery orange (AFTER WASH HAS DRIED COMPLETELY)
-Touch up the edges of your metalic object with mithril silver.
Another rust method which I like quite a bit is used to replicate pitted metal. Get out your rust color (orange!), thin it, take your tooth brush (okay, not *your* tooth brush, but a tooth brush no one will ever stick in their mouth), dip it in the thinned orange then run your thumb across the bristles so you get little pock marks on your metallic object.
This is really messy and you should test run it a few times before you start flicking orange paint on your awesome new mini.
PS- the flicking method is also a good way to make a star cloak