Steve, I shoot mostly acrylic. I am burning through my collection of Polly Scale and I'm starting to shoot their equivalents in Model Masters. For weathering I use Grimy Black, RR Tie Brown, or Roof Brown most of the time. I also use Rust sparingly. I'm not sure that one made it over to MM.
Dave, like you, I always start with Grimy Black on the couplers, then I give them a pass of brown or rust. Sometimes I just leave them Grimy Black. The last step I do with weathering is dry brush everything. I sometimes use a little rust dry brushed on the trucks and couplers. I've never put enough paint on the Kadees to gum them up though.
Here's a couple comparison shots of Kadees vs. Atlas couplers on the X29s and 8000 gallon tankers:
Heres the Atlas p/n I am using:
