So, more tank today. I picked out some other details on a couple of the guns in black. I also tried to work with the 'rusty' spots. The rust powder did not come off with brushing or rubbing. Trying did kinda push the powder around some, giving one side of the tank a sort of rusty-dusty look. :p I found that using a wet brush would sort of 'wash' some of it off, but not very well. So, I ended up painting over the areas I didn't want all rust. It's not great, but arguably better than leaving it as it was.
Now, the black powder brushes off if I look at it funny. They are different manufacturers, so maybe that's the problem. Next time I'll have to try a rust colored powder from the same set as the black and see if it goes any better.
Tomorrow I'm going to do the tank treads using a recipe from CMON that uses only weathering powders to get muddy treads. Then I'll have to wait to get some spray fixer on them before gluing them on the tank.
Oh, I was supposed to put oil on some of the joints as well. I forgot. I'll get back to that tomorrow. It can also be done with weathering pigments, but I've got a special product from Tamiya that I already know does really nice oil and grease effects, so I'll be using that.