Basically what I have done (as there are so many orks in an army) is use product to get the effect im going for. I aimed for gaming standard slightly above, as when you field a horde army the overall size is impressive if fully painted!
Metal scratches - Used foam out of blister and dry sponged bolt gun metal
red - Mechrite - dry brushed with blood red (when you going for dark muddy weathered efferct highlighting in the normal way doesent match the ffect)
Glyphs Chequers - transfersapplied on the base red before any weathering is done
Final effect - Badaab black and lots of! - if you do this effect its almost like dipping - apply loads over the red an metal areas! Not cheap but gives the effect im looking for
alternative - Dry brush the orange foundation on metal areas you want more ages and wash with dev mud
Final - Tamiya weathering pastels, used the rust one to go around the areas i wanted to look mucky/rusty. You need to seal though as it does rub of if you dont!