ok, I think I must be missing something fundamental. here's my stab at a directed wash, as per what winterdyne described:
I used about one part devlan mud to 3 parts water. on the knee, I started the application from around the middle of the knee and dragged the brush down to the joint. that's 5 layers and it's barely visible to me. I also didn't end up with a hard shadow in the joint, so I guess I'd have to add that later.
I made a few more attempts on the back side:
on the bum plate (never thought I'd say that...), I used about 1 part devlan mud to 2 parts water and started from the center of the plate and pulled out towards the edges. that's 6 or 7 layers, I think. I tried glazing the middle area with
GW's new waywatcher green glaze, but it didn't do much beyond dry patchy :( I added the edge highlight just to see where things stood. I think it looks better in the photos than in person. in person, it looks more like dirt or weathering; it doesn't feel dark enough to be shading.
the model's right leg was done the same way as the bum plate, including the glaze. the left leg was done with my old method: a single application of un-thinned devlan mud straight into the recesses. it's the cleanest shading on the model so far, but, of course, it lacks the gradient effect I'm trying to learn. I just slapped it on there so I'd have something to compare against.
so... where am I going wrong?