Well ... the people here who have had to repair rust know its not pretty ... fact of hte matter is they hold moisture which == rust .
I think its common sense
Hammerite spray is a type of spray paint that forms really hard and textured ... make sure you get way up in there with a fender brush (or a wire brush if your daring) and clean off ALL dirt .. if your daring, get rid of the paint up in there, wash it really really really well 3 times over until no dirt comes off then give it a sand and paint it with some rust inhibitor (por15) and then spray overtop with the good undercoat / hammerrite spray ..
thatll leave a rock chip roof surface that'll keep moisture out and protect the metal in the tire well ... if you want to go hardcore, take off the bumper and tire to do it, leave it jacked up and make it a week long project.. the longevity of your fender wells will thank you ..
here the city is really problematic for rust because of so much salt on the raods in winter.. check my sig pic for the rust and how bad it got ...
You know that lip that wraps inside the tire well that you can see from the outside ... it has those rubber things on that jjust slide over the lip ..... the lip tis held onto rusted completely in areas ..
as well where the bumper is held on, that bolt hold was rusted through as well ..
the key is to keep all the dirt cleaned off and allow it as much air possible so it can dry when it does get wet in rain/snow ... but its hard to do it in winter (taking off tire, using fender brusH) and theres no room without jacking up the car so ... dirt stays, salt stays, water stays ... rust seeps in .. and out the outside .. and your left with a fat bill to repair it ..
boy that was kinda long but .. yeah, the best advice, take it out to allow air to dry everything out, wash your fenders often and the hardcore is the undercoat as mentioned above.