Thanks Cheesy ;D!  Thanks Q! 

Here's the saucer masked off with blue painter's tape:

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IMG_4574 by trekriffic, on Flickr

And after airbrushing the rust "ring". Not a ring so much as an arc really:

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IMG_4575 by trekriffic, on Flickr

Tape removed. The flash makes it look darker than it is:

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IMG_4576 by trekriffic, on Flickr

I need to spray the nacelles and the secondary hull (need to mask the windows) with hull color too so I'll wait until I'm ready to spray them before I do the overpray on the saucer.