I wanted to get the scenery from Kill Team Octarius done quickly, so painted it with the (relatively) quick method. It is not perfect and I won't be picking out every detail, but I think it looks great on the table.
Dirty Rusted Buildings
First, prime the terrain. I gave the building a quick blast with   Chaos Black  from underneath and then sprayed them from the top and sides with   Leadbelcher . I don't plan on giving the inside of the buildings much attention, so I will keep these areas in shadow.
Spray a dirty-yellow roughly, over any areas where water and grime may settle. I used   Plague Brown . For this stage and the next two, keep these paints very thin, to give them some transparency. If you are using Games Workshop paints,   Zamesi Desert  is pretty close.
Being slightly more precise, use a saturated light brown to cover a majority of the same area again. This time be a little more precise. I used   Beasty Brown  for this step. If you are using Games Workshop paints, I would recommend   Mournfang Brown .
Trace rivets, panel lines and corners from the previous step as closely as possible. This time use a red brown, like   Dark Fleshtone . From Games Workshops range,   Doombull Brown  is the equivalent.
Spray the whole surface with a mix of washes, to get a grimy colour. I used equal parts   Nuln Oil ,    Seraphim Sepia ,  and    Agrax Earthshade . I gave it a couple of passes and adjusted the ratio to get the shade I wanted.
Drybrush with a light metal, just enough to pick out edges and rivets. I used   Necron Compound  with a large drybrush.
I felt that the overall tone of the metal was still too bland at this point, so wanted to add some variety. To do this I picked out the random pieces around the bottom and bits that looked like new additions with   Steel , but   Leadbelcher  would have been fine too.
Now we re-unify everything with an oil wash. I first gave the buildings a thin coat of gloss varnish and then slapped some thin Vandyke Brown by Winton all over the metal. This darkened the recessed nicely too.
After 2 hours of drying I roughly ran dry make-up sponges all over the model, picking up any excess from exposed areas.
Now dry brush everything again with   Necron Compound  or another light metal, reestablishing the light areas from previously.You could call the buildings done at this point and have something that looks OK on the table. If however you wanted to add some additional quick weathering to the rusted metals, check out the steps below.
To break up the uniformity of the buildings and add some visual confusion, I decided to chuck in some different colours. I wanted to stick close to the games workshop scheme, but decided to do heavily desaturated versions and avoid typical ork clan colours.
Dark Rusted Metal
Rusted Whitewashed Metal 
Oxidized Copper
Aged Cream Paintwork
  Aspiring miniature painter and biscuit enthusiast. Here to share my hobby and help where I can.