Rusty discs

The rotors are generally cast iron which basically rust on contact with air. This acts as a semi protective coating in its own right, think of the old browning process that they used on black powder firearms. Nothing you can do to stop this short of pulling off the rotors and painting them, use a very light dusty coat of pot belly stove paint. You then have to put up with dodgy brakes until all the paint has worn off the rotors and pads.
As long as the rust is just surface its not a problem and will do no harm. If however its getting to the deep "pitting" and flakeing stage it may cause weak spots and new rotors may be needed.
If its just an appearance thing don't worry about it.
Alternatively have some manufactured out of stainless steel.
Rust and the resulting looks was the major reason motorcycle manufactures went to stainless steel for brake rotors, cast iron is still more durable and gives better feel and IMO better brakes.

WD40 weekly?