I started with three Crystal Light containers and a CD spindle. Research showed these types of containers are typically white. Very white, if well maintained. I decided early on that mine would be weathered and rusty.
For railings and ladders, I went with my trusty Granny grating (cross-stitch mesh). It's easily used in this capacity, as well as for grated floors. I cut out railings and ladders to fit and then hot/PVC glued them to the structures.
Here are the silos, again with ladders, railings and a pirate.
After that dries, I primed the whole collection with flat black spray paint. When that dried, I over coated it with flat white spray paint to form a mottled gray. I was sorely tempted to leave them that color, as they wouldn't stand out too much on a battlefield, yet they would look presentable. I decided to try to whiten them further.
Here are the silos after a couple more coats of flat white spray paint.
How to weather them? In my mind, I see rust as orange and brown. I used orange ink first, which was a mistake. Too bright! I dampened that hue down quite a bit with brown ink, but it was still too bright.
One trick I learned was to apply a semi-heavy layer in an area and then 'pull down' with my finger to create the impression of a liquid flowing and then drying out, leaving a stain on the white surface. Thank you Bob Ross.
I then hit the heaviest orange areas with brown paint. This did help, but it's still pretty orange. I consider this fuel tank to be corroding and not given maintenance.
Having learned my lesson with the fuel tank, the silos received a different treatment. I forewent the orange altogether and stuck with brown ink. To me, they look like they are dirty, but not quite to the really rusting stage.
A quick gloss coat of sealant later and there you have them. A quick, easy and cheap set of fuel tank terrain.