This building was one of our first attempts at using model railroad building shells as gaming terrain. It had problems from the beginning. The worst problem was that the building is just a bit out of scale, it's too small. So I did a really basic paint job on it and let it go. The windows were molded plastic pieces, so I painted some of the panes black to simulate that they've been broken out. The only really "advanced" technique was to use black pastel chalk on the chimney to simulate soot.
The side had a nice loading dock and the back had some neat sliding doors, so those were embellished just a bit with some dry-brushing. Basically, we allow the building in the game to just kind of "hint at" industry. It really doesn't have any other use from a tactical point of view, so it continues to serve it's purpose: as cover and a place to hide.
The interior, such as it is. There's nothing to it, just enough to allow figure placement for tactical purposes. The foam board was glued to the sides with a crafter's hot glue gun and occasionally repaired with a glob of green putty.