So, I've started weathering the tank with weathering powders. I'm going for two effects: soot/smoke stains on some of the gun barrels, and rust around some of the rivets.
The powders are going to take some getting used to. I took a class on how to use them at ReaperCon (thanks, Martin!), but, well, seeing a demonstration vs. doing it hands on is a little different. After some fiddling, I got the soot/smoke looking the way I wanted. The rust...still working on it. I just wanted a bit of rust in the crevasses; what I've got is the whole areas with the rust color on them. Theoretically, the powder should brush off, but that wasn't working out well for me. I'm giving it some time to set, because some of the rust areas I did with the pigment mixed with a little alcohol. Tomorrow I'll come back and try to rub off the high spots to get the effect I'm looking for.
Also, the powders make a big mess what you work with them. Next time I need to put down some paper first.