Looking at that, other than the bashed up metal, the scratches and rust appear to be 100% painted effects. The rust streaks appear to be painted using weathering powders - essentially just paint pigment as a powder, which can be carefully brushed on to create effects. It's commonly used in scale modelling applications, so have a look around for weathering model trains and such. The washes on the model are probably the same powders mixed with a little alcohol.
Have a look at
http://www.ipmsstockholm.org/magazine/2003/01/stuff_eng_tech_afv_weathering.htm for an example of using the powders for dust. It's not a huge leap to use it for rust instead.
The scratches are painted on. Essentially you paint a scratch in black, highlight the middle with brown leaving a black border, then paint the bottom edge of the armour with a lighter colour to represent the light hitting the divot. if you can read French, check out
http://neomodel.free.fr/index02.html . Personally, painting the many scratches by hand and making them look random is difficult, so i often use small pieces of sponge, tweaked a little with some brush painting to make my scratches.
Take for example this little guy here, who was a test gaming-quality model for a potential Nurgle army that never saw the light of day.
You can see on him the scratches, particulary around the legs and shoulder pads, which were created using a sponge with a little black paint on it (and with most of it wiped off). Some scratches were then merged with a brush into larger chips. The larger black areas were highlighted in the middle with brown, and the bottom edges highlighted with lighter green. The sponge also works well to apply blood to his knife
Lastly, do not underestimate the power of s standard pencil. Run along the edges of a surface, it's a quick and easy way to create the appearance of scratched paint.
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