@Matroskin, gamedev: Been trying out some different approaches on the wall and solved the problem now.
Easiest way to solve it, was to simplify the brushes and it all works now, thanks for the tips =)
@nick2730: just like jmiles said, it's just decals. That decal is used a lot and it doesn't even has an opacity mask, it just rust on a white background that is set to blendingmode -> modulate.
@jmiles: i'll look in to the dusty particles asap, good idea
started to work on a smoke particle from on of the pipes, but haven't figured out how everything works yet
@Snight: good idea, looks much better with a little blue tint
here's some progress screens: