UPDATE: July 11th.
Hmm. . . I kept looking at my rust paintjob and decided I wanted to tone it down a bit. A couple of recent members posted some pics of real grenades, and they didn't look nearly as rusty as I had made mine.
So I went back with some of the base metal color that came with my rusting kit and thinned it waaaaay down and sprayed it all over the frag body. I blotted it a bit, and after it was dry, I went back with a small sponge and used this to "drybrush" some contrasting gun metal to all the outer surfaces.
I'm happier with it now.