After reading numerous threads about rust/corrosion in the rear wheel arches, I decided to have a look at mine. My roadster is always garage kept, seldom driven in the rain, never in any snow or salty roads.
I jacked it up in the driveway and removed the rear wheels.
Lo and behold, I found about a pound of black sandy dirt in each arch !
I flushed with hose, then used a fingernail type scrubber brush several times with flushing in between.
Now they are pristine !

If you plan on keeping your car, I advise you to have a look.
Also excellent opportunity to clean up all the suspension parts and fender wells, as well as the inside of the wheels.

( probably some WaxOil would be good too, but I didn't have any )

BTW : For those that haven't had the wheels off for a while, or just plain forgot : the bolts are 17mm, the recommended torque is 82 ft/lbs. The caps are keyed and should just snap on, don't grab that rubber hammer !