Getting close now...
I used an oil dot filter to add streaks of dust and grime to the knight. There are lots of good tutorials online so I'm not going to drone on about it here (search oil dot weathering tank on Google). The hardest part about this technique is getting up the nerve to try it, so practice on a spare bit of kit and you'll find out how easy it is. If you get too much grime on your model, white mineral spirits on a clean brush will take it away! After treating the whole model (I used dots of Burnt Umber, Burnt Sienna, Yellow Ochre, ans a (roughly) 25:75 mix of Cadmium Yellow and White. The effect is very subtle and adds nice tone variation across the armor and enhances the recess washes. After giving this a few hours to dry, I went back with the Burnt Sienna oil and picked out a few areas where I wanted more pronounced stains (see the vent panel and hatch handle in the pic below).
I also used black oil paint and thinner to add some grease and oil stains around joints.
Whew! Last steps in a few days. I am going to add a few purity seals, blacken the muzzle of the cannon, add some mud and dust with weathering powder (mainly around the feet and lower legs), and add some soot (around the vents). Then the big guy will get a coat of matt varnish to seal in all of these oils and powders and get rid of the remaining shine from the gloss coat. Finally I will touch up small areas like the cockpit window and eyes with gloss varnish again.
The base still needs some work as well, but I'm in the home stretch and incidentally starting to assemble some more knights! My plan is to have a group of three Hawkshroud Imperial Knights, one converted Hawkshroud Imperial Warden (more on that soon), and one Freeblade Imperial Knight where I can exercise a bit more creativity.