My take on the challenge.
Final! - Added white mineral deposits (that's what I think they are in the reference: http://www.textures.com/system/gallery/photos/Windows/Industrial/15601/hotlink-ok/WindowsIndustrial0068_shared.jpg)
6th Iteration - Started refining the colours.
5th iteration. Near where i want it to be, I balanced the wear and masking a bit.
4th Iteration. Added cracked glass and wire mesh.
3rd Iteration - Started wear and tear.