I know DoomsDay Machine Ch4 took awhile to complete. I had a lot of it done, but between school, FFXI ONline (which I quit), mass problems and trying to visual the set kept me from working on it. This is just how I do things (as usual).
First off, the set had to be modular. Yeah, one can do a WHOLE cityscape but when you need to jam your camera in weird, tight places, you have to be able to move things around, take walls down, MOVE the sets to a better shooting location etc.
Basically, I started cutting out what I figured were war-torn walls and the scale is not done with a ruler, but by using different sized figures. As you can see here, I'm using ResoDuke and TunnelRat. It'd be pretty lame if I cut the window holes too low or too high, right?
I wanted to give texture to the walls. So I grabbed my good old lil cup of spackle and spread it on the foamboard. Thicker in some places, letting veins happen, etc... It looked pretty neat. It also seemed to protect the foamboard a bit in that, it didn't wrap as much and the paint gripped better.