If you use the gummi, I'd just use gentle pressure, and if you dull your edges a bit, sharpen and polish 'em up...
if it's a light rust, i'd just sharpen as normal, but start your more coarse grit with something more durable... I wouldn't want to get the diamond stone all mucked up with rust... maybe an arkansas stone or something cheaper and more durable (alum-ox stone, very light pressure, maybe?)
If I were you I'd try to figure out WHY my edges are rusty, and try to keep them from getting rusty... throughout this whole season, I haven't had any rust at all on my edges.
Disclaimer: I'm still rather new at the tuning thing, so a 2nd opinion might be in order.