Sorry, only just seen this thread.
I've experimented with various methods of weathering, some more successful than others. This particular loco was weathered with graphite paste. It's sold in tubes and used for cast iron fireplaces. Just brush it on then wipe most of it off with a soft cloth. It's very good for getting a nice steel look to the valve gear. The downside is that some residue can come off on your fingers. To reduce this I give the body a dusting with weathering powders. This helps set the graphite paste and gives some nice variations in shade. I used the same method on one of the Forneys on my website.
If you decide that it looks too weathered for your taste you can always remove the paste by washing it with white spirit. Small amounts will remain around rivet heads and such and it will look like a clean but used loco. You can do this at any time as the paste never fully dries as paint would. It's an almost totally reversible process so it's worth a try.