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 !

 

It is best to clean then you have been out in the rain as that softens up the muck held in there. Do not forget the fronts, a liner is there but that just hides the muck that has got behind it. There are about three spots where the build up can be felt.
Do not forget the bottoms of the doors either behind the top seal.

 

But that would be only once every three years or so ! My roadster is allergic to rain, so we avoid it like the plague.
The last time it saw rain was on the Eureka Springs trip.
I was so tired when I got home that I didn't wash it right away, it sat in the garage and cried for a whole week, too ashamed to go out in public !

 

When it is dry out the dust and dirt is no problem, when it in the wet the mud and dirt gets thrown up there. So the best time to get it off is while it is still wet, this way the car dries and there is no hidden wet mud to start corrosion.
When this dirt dries the water bounces off it and it a hell of a job to remove. So hose the underside while it is still wet, I don't but do as I say not what I do. LOL