Luckily, Welf VIII. from this forum requested some in-progress shots while I painted the miniature, so I can show you how I did the rust effect on the skeleton's armour:
VGC=Vallejo Game Colour
VMC=Vallejo Model Colour
I always thin my paint with a 1:6 Mixture of Water/Future Floor Wax.
parts primed white(!) sprayed over a black undercoat. I find it easier
to define the general direction of light shining on the miniature.
VMC #828 Woodgrain (Transparent) wash over metal parts:
Sorry for the blurry pic...
highlight using a 50/50 mix of VMC #828 and #851 Deep Orange. I applied
the mixture as a wash. The trick with rust is that it works the
opposite way that other materials. Liquids tend to collect in the
deepest areas of a metallic object, so the lightest parts should be in
the recesses. So it really should be called 'deeplight'...
Curses! Blurryness again!!!
I added some more orange to the mix and applied it to smaller areas deeper in the recesses:
VGC #38 'Scrofulous Brown' to give it a yellowish hue and VVMC #70815
'Basic Skintone' to make it more opaque was added into the mix:
...and a different view:
added some metal areas where the rust has chipped off in order to break
up the whole, erm, orangyness. I used VGC #54 Gunmetal and VGC #72053
chainmail for this: