I'm running low and haven't been able to find any. The guy at my LHS said it might have something to do with the chemical content(Alchohol?)
OK, OK, I'll stop drinking already! aussie ............................................not.
There are more than enough different methods of weathering, Airbrush, drybrush, chemical, pastels, etc. etc. Each is as good as the next, it's a matter of which technique is easiest for you to use.
Whichever, there is also a need for observation. Look, and see how things weather, and to what degree. You will see boxcars (RBOX, Railbox comes to mind) that look like they are fresh out of the factory, and then, the same paint, and car style that looks like the third res******** from the scrap heap!
Buildings will weather according to their construction materials, and age in the location. Rolling stock weathering will also vary with use, age, and most often visited location.
Variety........amount of weathering, types of trees, colors of grass/leaves/weeds, variety is the key to most realistic appearance.