Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i, 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
If it's only light surface rust (which it undoubtably is) just let it go. It will wear off very quickly during your first, very noisy, brake application. Anything you could apply to keep the rust from occurring will do very bad things to your braking system. Take a look at car brake rotors after a heavy rain. You'll see every one of them with surface rust. It's not that big of a deal.
Alternatively, and for the sake of the rest of your bike, a simple cover would keep the majority of the moisture off the bike. Your brake rotors rusting should be the least of your concerns honestly.