I wait until the ballast glue is dry to apply final weathering.
One of the tracks here is heavily used enough I figured it should receive some greasy weathering. I used slightly diluted craft acrylics applied with a stiff brush.
Rob Spangler
Just tried this for another reason. I have a road that was done with fine ballast and it looked too new, so I splashed an india ink wash I use for wood (some brown and black mixture with alcohol), worked great and easily controled.
For oil streaks where oil has leaked from a moving train, take an ordinary #2 lead pencil and using a rail as a guide rub the pencil back and forth over the already ballasted track the distance you wish the leak to show. the more you rub the darker it will get and look like oil that has been there for some time. I also like Howard's idea for weathering also and will try that on my layout this week. Thanks for that tip.
In good train stores and shops like Michael's, look for a jar of General's chorcoal powder. One jar can do several large layouts. Apply with a wide flat tip brush, and there is your perfect ballast/track weathering. You may vacuum it off later of if you prefer just hit with a clear flat spray and wipe rails just after for permanence.
HZ
The main reason against it is that matte medium needs to be uniformly applied over all your ballast. But the oil isn't going to be uniform like that. It's going to be more of a stripe down the middle.
Steve S
I ballast my track with dilluted matte medium applied with a medicine dropper. One thing I've been wanting to do is weather the ballast in between the rails with an India ink wash to silulate oil and grease. Is their any reason I couldn't add a few drops of ink to the diluted matte medium?
Modeling the Pennsylvania Railroad in N Scale.
www.prr-nscale.blogspot.com