For purposes of this tutorial only, I took a termminator that was already basecoated white. There is no specific reason why it is undercoated in white. It just was.
To create the best view for this effect, I decided to create a light toned color for the armour. In this case, a blueish grey. Beware, if you want to try this on WHITE (really, almost pure white), much of the effect later on will be undone as part of it is a white highlight - as you will see later on:
Now here is where I am missing a picture, but I think I can describe it. I used Dark Flesh and dabbed with a rough brush on the right side on the rims of the armour parts. On the left side, I used a sponge of sorts (it actually is the same stuff models are packed in in blisters), and stamped the same color on. it makes for a more realistic effect, but is also harder to maintain, or even control.
After this stage, I took Fiery Orange and stippled a bit inside the blotches of Dark Flesh. On the next picture you can see those stipples, as well as the difference between a brush and a sponge.
Now comes the harder parts, where you have to use a small brush and do some fine lines on exactly the right spots. I took the shading lines (Chaos Black) first, but you can do the highlights first too.
Now what I am about to explain is gonna be a complicated story, but when you see it, it all makes perfetly sense (Ihope). What you are painting for shading, is the shade where the top part of a blotch of rust is covered with the armour's original paint. So in short: put a fine line of black along the top parts of the rust spots. But there is one exception: if there is no paint above it (for instance because it ends in the rim of the armour), then leave it: Here are some views:
You'll see when you add this last part, you really get the depth of the weathering in. If you can do this really neat and correct, you can paint indentations, chips, cuts and scrapes and have people ask you what tool you used to make that battle damage - only to reveal it was all painted on!
* you could also stipple a bit of a metallic in the biggest chips, to have the metal show underneat the rust.
* It should be aparent by now why this would lose a lot of it's effect on white surfaces. It can still be done, but it might require another apporach of the dilemma.
* Even really small dots can be made to have depth, if you use a brush small enough, and just plae a white dot under and just left (or right) of the rust dot. Make sure both dots border eachother - don't leave a gap!
* For better effect, really take a good look at WHERE rust realistically might appear, and HOW it behaves. Use some dark brown washes and draw a few water leaks starting from chips, to make it more weathered, old, worn.
Oh: and please, never hesitate to leave a comment, even just to say hi! I might just say hi back :)